glitch-soc's public.js was a bit out of date, and code was put inappropriately
to the common public.js

This fixes updating the profile preview when changing display name.

Upstream's implementation has been merged a while ago and is the preferred
way to set fields, as it is the only one compatible with upstream and is
more user-friendly.

This commit deletes the legacy glitch-soc FrontMatter functionality in order
to clean up the code and make it easier to maintain.