Show default tag for PMs when none chosen.
Switch edit/compose button on PM list.
Parse whether PMs were forwarded.
Show replied/forwarded/seen on PM list.
Empty tag for PM compose view.
Post icon picker looks like @ridilla's mockup.
Fix crash sending PM with no subject.
Extract retheming functions.
Dark theme for PM list.
Dark theme for PM compose view.
Formatting and docs.
Post icon picker dark mode.

Also moved some stuff out of picker's init. No point doing more than we have to
here.