Fix mentions, room joining for guests & add same-room visual clue

[7PH]

• Prevent user mentions in private rooms where mentioned user has no access to
• Fix guests not able to join public rooms
• Add visual clue to indicate what connected users have at least one connection in the same room than your current room

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

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