As suggested in b387f4a1c1, only steal
emoji with alphanumerc, dash, or underscore characters.
Also consolidate all validation logic into a single function.
===
Taken from akkoma#703 with cosmetic tweaks
This matches our existing validation logic from Pleroma.Emoji,
and apart from excluding the dot also POSIX’s Portable Filename
Character Set making it always safe for use in filenames.
Mastodon is even stricter also disallowing U+002D HYPEN-MINUS
and requiring at least two characters.
Given both we and Mastodon reject shortcodes excluded
by this anyway, this doesn’t seem like a loss.
af041db6dc