Mentions - make them case insensitive

Mentions are currently case sensitive so @tyro will work but @tyro will not. This is a problem as:

• mixed case mentions are still styled as valid even though they aren’t
• Mac OS automatically capitalises the start of names making it harder to type all lowercase mentions

Solution: Make mentions case insensitive! NB. We won’t change the original markdown, rather we’ll downcase all detected mentions on the backend when parsing them for follows.