Safe activities are:
- those which are not visible outside current transaction (IOW, queued
  in activity buffer)
- those which are already marked as failed, as no activity node will try
  to process these
Other activities may already be running.
Note: deleting even failed activities may violate activity dependencies
(other activities may be waiting on the ones being deleted, even those
not spawned yet), but if this is an issue then some other code should
take care of these as well.
Also, rework a CMFActivity test checking that activties already-spawned on
a later-deleted context get discarded instead of being executed: previous
implementation was working around the activity deletion removed in this commit.
Also, test indexation/unindexation parallelism.

uids are a way one can signal that different objects (from a ZODB point of
view, and hence an oid point of view) are actually to be considered as the
same objet (from a higher abstration point of view). For example, another
variant of the same object, as imported over an older variant.
In turn, this allows extending the protection to activities spawned from
brains, and not just from live objects.

To save a bit of time every day (and save us from typos)

As now ERP5 is mainly used though the web interface, the use of the save action is nowadays very limited

ZODB Components: Preparation of erp5_base migration from FS: Fix pylint undefined-variable errors: Remove dead code.

keywordsplitter() has never been defined so this code has been broken
since its introduction.

ZODB Components: Preparation of erp5_base migration from FS: Fix pylint undefined-variable errors: get_transaction() removed in ZODB 3.6.

ZODB Components: Preparation of erp5_base migration from FS: Fix pylint undefined-variable errors: ResourceLockedError.

We receive feedback from users saying that "when I view a mail message in event module, I see ugly html source", this was because user had set preferred text editor to monaco. Improve a bit monaco integration so that we display something better than code in a `<pre>`  for non editable editors.

We had another feedback that when using CKEditor as preferred text editor in old UI, users can not click on links when viewing an html Mail Message. But it works in new UI. I did something minimal so that this looks working until the migration to new UI is complete.

/reviewed-on nexedi/erp5!1011

With the current architecture of cache manifest, it was required that
developpers change the cache manifests referencing a web page every time
they change a web page. In practice, developers were never doing this,
so we sometimes had issues where client keep using old version of
website even though a new version has been deployed, when after
deployment we did not change the cache manifest.

To automate the scenario of modifying the cache manifest, introduce a
post-upgrade constraint that will check that the manifest is newer than
all of the referenced pages. If that's the case, the constraint can fix
by modifying the manifest content, which also changes the modification
date of the manifest, which might be used in "if-modified-since"
negociations.

According to spec [1], cache manifest is updated if the HTTP responses
is not "304 not modified" and if the manifest content is not
byte-for-byte identical to the previously cached version.

[1] https://www.w3.org/TR/2008/WD-html5-20080122/#updating1

/reviewed-on !1009

Post upgrader constraint to automatically fix modification date and an upgrader bug-fix for an issue that's revealed in this new test.

/reviewed-on !1013

This tests had a test instance URL hardcoded and was working only on
this instance.

The default size of iframe is too small for CKEditor to display dialogs
properly. Increase the height so that CKEditor display dialog boxes
correctly.

/reviewed-on nexedi/erp5!1010

ZODB Components: Followup of 47b14fd4 making sure that there is only one Delivery to update (as it was before that commit).

Web sections or web sites displaying a default page should have
modification date after the modification date of their default page, so
that the "if-modified-since" requests can be replied with a 304 not
modified responses.

Upgrader detects that if some type have constraints and their container
type also have constraint, it's not needed to check again this type
because since checkConsistency is recursive when the container is
checked they will also be checked.
This fails when we have situtations like:

  X is allowed in Y
  X is allowed in Z

and there are constraints for by X, Y and Z.
This was raising a KeyError when pop'ing Z :

    Module script, line 50, in Base_getConstraintTypeListPerPortalType
     - <PythonScript at /erp5/Base_getConstraintTypeListPerPortalType used for /erp5/portal_alarms/upgrader_check_pre_upgrade>
     - Line 50
      type_list = constraint_type_per_type.pop(allowed_content_type)
    Module AccessControl.ZopeGuards, line 104, in guarded_pop
      v = guarded_getitem(d, key)
    Module AccessControl.ZopeGuards, line 83, in guarded_getitem
      v = object[index]
  KeyError: 'Z'

In fact, this code is wrong, because if Z is also allowed in W and W
does not have constraint, then Zs inside Ws will not be migrated. In
practice, that's probably OK for now, because we don't use migration
constraints on documents so much, most of the time we only do one
script that checks the constraint globally.

/reviewed-on nexedi/erp5!1003

Many typos in: a9d81b09

In xhtml style, clicking on <a> links in a non editable ck editor does nothing. It's possible to to use Ctrl+right click to trigger the popup menu and "open in new tab", but that's confusing for users.

Implement something minimal so that the event handler registered in ck editor gadget also works in html style.

by setting "readOnly" to true.
Monaco also supports applying its style to a <pre> element (like on
https://microsoft.github.io/monaco-editor/playground.html#creating-the-editor-syntax-highlighting-for-html-elements
) but it would be a too big architecture change, a readOnly editor is
enough.

We also sometimes display Mail Message or other documents that are
text/html, it's easier to manage this by content type rather than by a
mapping of portal types.

gadget_editor.js already supports a content_type key in state, but it was not passed by the field definition.

ZODB Components: Preparation of erp5_base migration from FS: Fix pylint unbalanced-tuple-unpacking warning.

ZODB Components: Preparation of erp5_base migration from FS: Fix pylint unused-import and unused-variable warnings.

New workflow actions added in 67f58b5b and b3dc32f5 thus adjust accordingly.