Create a mention @all to notify the current collectives

[Aveyron-RetD]

Is your feature request related to a problem? Please describe.
No shortcut to notify the collectives
Describe the solution you'd like
A clear and concise description of what you want to happen.
=> Like in spreed => When you are in a collective the @ALL to mention the entire collective member would be useful
Describe alternatives you've considered
Use spreed => @ALL to notify important information

Additional context
Add any other context or screenshots about the feature request here.