Forms: switched to div as a wrapping tag

Allows to include more complex block elements inside. Note that we would
be in trouble with `<p>` tags, as browser would take the included block
element out.
Admin app: provide default for app mount point

Also no need to store a reference to it in the namespaced object.
Cleanup: made admin API collection logic reusable
Cleanup: simplified passing props
Cleanup: removed some unnecessary props
Add filter parameter to show user translations for a month period

Fix #3431
Init stats cache before resurrecting obsolete items

Fix #3433
Get rid of contributors from the permissions drop-down

Fix #3430
Cleanup: switched to div as a wrapping tag

This was also preventing from using properly block elements inside.
Cleanup: use self-closing tag
Cleanup: minor
Forms: implemented FormSelectInput

Under the hood it's using react-select.
Languages admin: revamped functionality

Implements the new admin backend and UI for languages.