All shortcode regexes used [a-zA-Z0-9_] which excluded hyphens,
causing custom emojis like :GM-Chachi: to fail at every stage:
publishing (no emoji tag added to event), rendering (shortcode not
resolved to image), and detection (not recognized as custom emoji).
Updated the character class to [a-zA-Z0-9_-] across all 8 locations
in NoteContent, CustomEmoji, ComposeBox, and customEmoji utility.
Also added viewer's custom emoji collection as a fallback in
NoteContent so shortcodes render even when events from other clients
omit the emoji tag.
Closes#62
Mary Kate Fain
eba4121d63