[epic] replace JQuery widgets by VueJS components

[vinyll]

The following JS widgets should be removed and replaced by their VueJS components:

• [x] Integrate-Btn: check usage in territories project prior to refactor this
• [x] issues-btn: in progress
• [x] featured: to do
• [x] share-btn: done, delete widget
• [x] follow-btn: done, delete widget
• [x] ellipsis: to do
• [x] popover: Vuejs exists, insert directive where old widget is still used (see plugin / tooltip)
• [x] datepicker: extract style from package
• [ ] completers
  • [ ] remove unused completers (territory ?)
  • [ ] switch to pure vue components (like search)

To do last:

• [ ] jquery-validation (admin form validation)
• [ ] bootstrap-markdown-editor
• [ ] jquery-crop
• [ ] bootstrap:
  • [ ] home carousel (what do we use as replacement ?)
  • [x] modal: delete widget, clear usage in admin, contribute modal
  • [x] tabs: multi model search

Most of these widgets are located on /js/widgets and may be migrated to /js/components

[vinyll]

featured: https://github.com/opendatateam/udata/pull/527

[jdesboeufs]

Share and Follow seem to have been migrated. Cc @vinyll