Show a domain dropdown when multiple AD domains are configured

This integrates the functionality of the now outdated addomain plugin
directly into the authad plugin and makes multi-domain setups usable
without SSO.

See also https://github.com/cosmocode/dokuwiki-plugin-addomain
added missing language strings
Merge pull request #541 from splitbrain/multidomainad

Show a domain dropdown when multiple AD domains are configured