ref: ED021-1

User story

Is your feature request related to a problem? Please describe.
As a participant I want to be able to mention groups in comments and get group members notified.

Describe the solution you'd like
As a particpant, I want to:

• Mention any group in a comment, using autocomplete
• Members of a group are notified when I mention that group

Additional context
https://meta.decidim.org/processes/roadmap/f/122/proposals/12370

Does this issue could impact on users private data?
No, although it need to take in account the "Notifications" settings of the participant profile.

Acceptance criteria

• As a participant I can mention any group in a comment by using autocomplete.
• As a group member I am notified when the group I belong to has been mentioned.
• As a group member when I receive the email, the title of the email is "You have been mentioned in TITLE as a member of GROUP"