ERP5 Diff Tool is used to get beautified diff between any 2 ERP5 objects as well as different revisions of a single ERP5 object **(both in XHTML as well as renderJS UI)**

This would be helpful in:
--------------------------------
- ZODB History Tab to check the edit workflow history.
- Revision of the ERP5 object, for example: web pages, documents, etc
- Business Template installation

Diff Tool will also make it able to patch one ERP5 object from the diff *(for version 1, we only patch objects which are of same portal type)*. The current version of Diff Tool uses `deepdiff` python library which provides us with patch which can complies(not completely) with CRDT datatype, thanks to which patch application can be less painful.

Diff Tool is able to differentiate between the diff according to type of user, for example, we do not want a normal user to be looking at diff of some internal properties of object(for ex: workflows), but for a developer user, we are fine with it.

How we calculate the Diff of any two objects:
-----------------------------------------------------------
1. Convert the objects to their JSON format.
2. Use `deepdiff` to find out the diff for the 2 JSON objects.
3. Return a beautified JSON Diff

### NOTE:  This MR succeeds the MR related to create correct URL for History tabs: !695 !683

TODO:
--------
- [x] Add tests for beautified diff and patch.
- [x] Diff tool should be working in new UI.
- [x] Fix the pagination.

Example Diff Tool in both UIs:
--------------------------------------
![XHTML UI Diff](/uploads/846b6b7806939a64922e364d309c028c/Screenshot_2018-07-04_at_6.07.46_PM.png)

![renderJS UI Diff](/uploads/ae7021fbf9cf9f952a7d2d4044df8a78/Screenshot_2018-07-05_at_10.34.01_AM.png)

/reviewed-on nexedi/erp5!686

Fit test to new expected behavior.

This reverts commit 1b087745

/reviewed-on nexedi/erp5!718

This reverts commit 7439ab8e

/reviewed-on nexedi/erp5!717

/reviewed-on nexedi/erp5!716

Before, when solving a solver, parsing sub objects of solver_process was done directly synchronously
in solver_workflow. And we had the case where you have parallel transactions solving a solver,
each of theses transactions see remaining solver not in solved state, but once all transaction are
finished, all solver are solved. This could lead to the case where solver_process is never moved
to succeeded. Instead of using serialize (which may lead to conflicts), just use activities in queue
with a serialization tag.

/reviewed-on !714

/reviewed-on nexedi/erp5!697

/reviewed-on nexedi/erp5!713

of Free Subscription Request module and FreeSubscription_viewFreeSubscriptionRequest
view of Free Subscription

Extend`FTPConnector.listFiles` to sort files based on some server-side computed attributes of files.

This uses [`listdir_attr`](http://docs.paramiko.org/en/2.4/api/sftp.html#paramiko.sftp_client.SFTPClient.listdir_attr) method of paramiko. `sort_on` parameter is directly attributes of [`paramiko.sftp_attr.SFTPAttributes`]( http://docs.paramiko.org/en/2.4/api/sftp.html#paramiko.sftp_attr.SFTPAttributes).

At the same time, adds a minimal tests, that will only run if `testSFTPConnection_SFTP_URL` environment variable is set to an URI of a running sftp server supporting password authentication - which means that in our current tests infrastructure it will not run.

/cc @vpelletier @aurel @rafael 

/reviewed-on nexedi/erp5!680

/reviewed-on nexedi/erp5!711

And not the string "None". I thought ZSQLMethods were supposed to complain
when receiving None for non-optional fields...

manage_afterClone clears workflow history, which is precisely what
"duplicate" is trying to avoid.
This fixes a regression introduced in
  commit 7e810d5a
  Author: Vincent Pelletier <vincent@nexedi.com>
  Date:   Tue Feb 20 17:06:46 2018 +0900

      CopySupport: Factorise clipboard-related methods and code paths.

The Diff view in new UI will be using IFrames to display the diff.

Also, add changes in tests:

erp5_ui_test: Update history test according to use of diff gadget
erp5_web_renderjs_ui_test: Update tests according to new Diff view for Historical Revisions
testXHTML: Add Historical Comparison View to the list of listbox with no selection name
testXHTML: Simplify test_html_file and include way to ignore single html file

Language modification is done on another page, accessible from many places for now.

The original need to touch this comment is that it mentions now-removed
Folder_reindexObjectList script. But also:
- it mentions multi-ZEO setups, which are now discouraged if favour of
  using NEO
- it mentions pre-conceived optimisations, which can be wrong and hence
  misleading (up-to-date measures are needed before deciding what to
  optimise)
- it mentions a number of activity nodes which is nowadays exceeded
  without seeing any obvious issue caused by activity table deadlocks
  (which are nevertheless still likely an issue, just not yet breaking
  scalability)