i need it because use span for displaying icons in json form

/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)