Add a report user action

[bmarty]

When long pressing on a message, this bottom sheet is displayed:

We want to add a destructive action "Report user", below "Ignore user", with a thumb down icon (if available, else re-use the flag icon), that will send a report, with the EventId and the reason "Reporting user $userId".

No confirmation dialog before doing the action. We will reuse the confirmation dialog which confirms that the action is done, and which proposes to also block the user:

With a specific wording.