Implement @mentions support

[robert-virkus]

trafficstars

In a group conversation, mentions help to notify the mentioned person about a message specifically.

Compare https://github.com/coi-dev/coi-specs/blob/master/coi-client-spec.md#mentions-in-group-messages for the format.

The following parts belong to the mentions feature:

• OX Coi should detect and highlight such mentions correspondingly.
• The app should allow to auto-complete a mention. For that it should provide some sort of popup as soon as the user types in the @-sign at either the text start or when there is a blank-character (space, tab). When the user continues to type, the auto-complete options should be limited by the matching characters. The following participants should be available within the auto-completion:
  1. @all ,
  2. the group participants,
  3. any remaining locally known participants.
• Allow to tap on a mentioned participant to view contact options:
  • For contacts not part of the group: "Add to group"
  • For contacts now known locally: "Add contact"
  • "Start direct chat"
  • Do not show any options when tapping on a @all option

Internal bug tracker

[robert-virkus]

[Boehrsi]

Requires message content splitting. See internal bug tracker.