Allow instance admins to add custom CSS that will affect
every page of their instance.
This is done with a new CustomCSS instance setting that
works pretty much exactly like the Users CustomCSS property.
This custom CSS is then requested for every page load.
User styles/themes take precedence over this CSS.
* [feature] Email change
* frontend stuff for changing email
* docs
* tests etc
* differentiate more clearly between local user+account and account
* populate user
* [feature] User sign-up form and admin notifs
* add chosen + filtered languages to migration
* remove stray comment
* chosen languages schmosen schmanguages
* proper error on local account missing
