Move share dropdown into the right sidebar in the files app

[PVince81]

The share dropdown is implemented in core in OC.Share.showDropdown() and all related events and functions.

This is how to proceed:

• implement ShareTabView which will be a tab in the right sidebar (see Basic work for right sidebar #17656), a stub is provided there
• implement each section of the share dialog as separate classes
  • sharing with local users as one panel (View class)
  • sharing with link as the second panel (View class)
• write a class to manage shares, maybe OC.Sharing.Client (similar to OC.Files.Client here https://github.com/owncloud/core/pull/16902/files?w=1#diff-a6f5f0ecc4845f72e7fe0908a1f90f70R26)
  • this class must wrap the OCS Share API calls in convenient functions (even if we decide to keep using the old endpoints)
  • make use of the OCS Share API endpoints (see webUI to OCS Share API #17143)
  • this class will be used by the view classes when accessing shares
  • the alternative would be to use Model classes, and have the ajax logic there
• use handlebars for the templates (templates as hard-coded strings in the JS code for now to make them unit-testable)
• add JS unit tests for all these classes
• once the ShareTabView is merged, discuss/think whether to remove the old OC.Share.showDropDown in regards to other apps that might still be using it

@rullzer @blizzz @DeepDiver1975

[PVince81]

While doing so, also have a look at this slightly redesigned mock up, which could be implemented by the same occasion.

• use mock up from Design forward: Sharing dialog #3224

[PVince81]

As discussed with @rullzer and @DeepDiver1975, we will move the old OC.Share.showDropdown to the calendar and contacts apps respectively, to make sure they can still use the old dropdown as before.

[PVince81]

Fixing the apps is tracked here: #19214

This task is done. Note that it wasn't implemented 100% as described above

OCS endpoints will be implemented as per #17143