Add support for autocompleting wiki pages in other wiki pages,
issues, epics, MRs and work items.

- This MR Adds autocomplete support for Wiki pages in other
wiki pages and issues using the `[[` shortcut.
- Issues in projects can now autocomplete links to project
wiki pages.
- Epics can autocomplete links to group wiki pages.
- You can search for pages by title to find the doc page you want.

Notes:

- On group level items like epics and group level work items,
you can only autocomplete group level wiki pages. (EE only)
- On project level items like issues, MRs and project level work
items, you can only autocomplete wiki pages in the same project.

Changelog: added