Mentions: make `filter` behaviour match endpoint.

For pre-primed lookups, the regex was only matching matches at the
beginning of words, or after a space. BP’s Suggestions API has been
built to find matches of a value in any position.

The change removes the `^` character from the regex so that the
behaviour is the same. The highlighting still needs to be fixed.
Mentions: tweak highlight filter.

The old `highlight` filter always assumed that the match would be at
the start of a result, or after whitespace after a character.

As BP’s suggestions lookup will return results from anywhere in a
user’s name, this change adjust the regex to highlight in that
situation.