Epics autocomplete epic

• Autocomplete for epics should work when you type in the epic description and when you type an epic comment.
• It should work in a similar way as searching for an issue/mr.
• You should be able to search for epics in the current group and descendent groups only.
  • You can still paste a URL for an epic in an ancestor group or different group tree and the Markdown should beautify it.
• Epics start with &. So it should use that, instead of # in the examples below.