Add user menu that is opened via right-clicking/long-tapped on avatar
We can un-clutter our message context menu, which we had to share with user actions. (Even though we only had one so far.) I added one new user-specific action which allows you to quickly mention the user in chat. Otherwise you would've had to copy their username or use the completion menu.
It's convergent on mobile, it still has the hover indicator and it also is available through the AuthorComponent.
