Handle LMS-specific mentions behaviors

[acelaya]

We can either add a new sidebar embedder config option that the LMS will set to change some behaviors, or use some of the existing mechanisms we have to determine if the sidebar is running in the LMS context.

Some different known behaviors are:

• Replace rendered mentions by the display name instead of the username.
• Do not render at-mentions as links to the user profile.
• Show only display name in the suggestions popover.
• Do not show a mention popover, or at least do not show the username on it.

Additionally, unwrapped/plain-text mentions (for when opening the editor) should have some format which ensures whitespaces are preserved. This was not needed for username-based mentions.

These are some possibilities:

1. @<John Doe>
2. @[John Doe]
3. @John_Doe (replace spaces with underscores, although this could conflict if the underscore actually appears in the display name)

[acelaya]

I'm closing this as completed. The most important pieces are done, and the only thing left is deciding how to deal with the mention popover. I have created a separate issue for that #6910