Make sure mentions do not capture punctuation symbols after username

[acelaya]

Some of the logic in helper functions, when detecting if the caret is overlapping an @mention, or when searching for @mention patterns in a piece of text, are including punctuation characters in the mention. We need to avoid that.

An example, for a text like Hello @user, how are you?, the mention that we detect is @user, instead of @user.

Rob suggested applying a regexp like the one used by the backend: #6797 (comment)