[IMPROVE] Mention autocomplete UI and performance improvements

[rodrigok]

Proposed changes

This implementation copies the ids of the rooms where the user is part of to inside the user's object allowing direct queries improving the performance in replace of the required lookups. It allows us to list the members of the room first and respect the permissions to prevent users to list non-members.
It doesn't copy the rooms of types d and l since both are largely used and contains few users, so the code uses a different approach for them.

Issue(s)

How to test or reproduce

Screenshots

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves a current function)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Hotfix (a major bugfix that has to be merged asap)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have added necessary documentation (if applicable)
• Any dependent changes have been merged and published in downstream modules

Changelog

• New setting to configure the number of suggestions Admin > Layout > User Interface > Number of users' autocomplete suggestions (default 5)
• The UI shows whenever the user is not a member of the room
• The UI shows when the suggestion came from the last messages for quick selection/reply
• The suggestions follow this order:
  • The user with the exact username and member of the room
  • The user with the exact username but not a member of the room (if allowed to list non-members)
  • The users containing the text in username, name or nickname and member of the room
  • The users containing the text in username, name or nickname and not a member of the room (if allowed to list non-members)

Further comments