Tilde Friends

* Added support for showing, creating, editing, and RSVP-ing to gatherings. * Crash fixes and performance improvements. * Refreshed the wiki app user interface. * Made the app launcher the default view. The ssb app can be set the default again by long-pressing it. * Added a README.md to every stock app with a brief explanation. * Fixed an issue where invalid messages were sometimes accepted. * Update CodeMirror, libbacktrace, musl-libc, prettier, and sqlite.