Filter issues and MRs and users while referencing them

Description

It should be possible to filter issues and merge requests by title while referencing them in the editor even after typing a space character (after typing a #, a ! or a @).

Proposal

Add a search text box to the suggestions drop down like select2 or the filter for issues and MRs (below) has one at the top of the options list.

Spectacle.n21296

Example pictures

This:

Spectacle.S21058

can look like this:

filterable_issuereference

Edited by 🤖 GitLab Bot 🤖