Administration

Implements administration functionality as described in #16 (closed). Things to be sorted out:

• Some tests and validation edge cases are missing. These will be delivered tomorrow.
• We had the problem with all admin's e-mail addresses being shown on error pages being not a good idea. Consequently we would then but have to add some column contact to the system_settings table and corresponding DTO attributes, of course. Does anything break if I apply this schema change tomorrow? On this occasion I could also change the default e-mail address regex from .* to at least .+@.+.

This PR should be merged as quickly as possible, optimally tomorrow afternoon, because it contains a nice testing framework for data gateway classes that will be useful for all other work packages. Therefore: Please review, review, review 😉

Closes #16 (closed)