Adjust mentions bbcode to better distinct between user and group

The mentions bbcode is currently of this format:

• user: [mention=u:3]test[/mention]
• group: [mention=g:3]testgroup[/mention]

This is a bit different than previous versions and might seem un-intuitive to some.
Suggested changed versions could be:

• [mention user=123]someUser[/mention]
• [mention group=12]someGroup[/mention]

Or:

• [mention user_id=123]someUser[/mention]
• [mention group_id=12]someGroup[/mention]

This is partially inspired by how we define quotes:

[quote=notificationtestuser post_id=18 time=1621773761 user_id=55]This is a test post[/quote]