Enhancement: Theme Manager

*Warning: This pullrequest is only opened for deeper testing.*

What does this change? It refs to https://github.com/Codiad/Codiad/issues/422 and https://github.com/Codiad/Codiad/issues/407.

Currently, this pull should *not* be merged, but as I can not find any issue, I would be happy if someone else could also test it and share me your opinion. Themes can be changed during the login and the default one is defined in config.php as well. I think that should be discussed in https://github.com/Codiad/Codiad/issues/407 as I orginally planned to define theme settings per user.