Add two very basic system tests

[oneiros]

Fixes #367

@rwaffen You might want to have a look. These are the two cases we talked about, but if you look at the code I think you will find that adding additional cases should not be too hard.

The only "real" obstacle here was that I had to create a little helper method for the "slim-select" widget we use for dropdowns. But this is solved now and can be reused easily.

[bastelfreak]

@oneiros are those also executed by bundle exec rake test:all or do we need to adjust our CI config?

[oneiros]

are those also executed by bundle exec rake test:all or do we need to adjust our CI config?

They are! No adjustments needed. Only bundle exec rake test or the more modern bin/rails test without the :all will omit the system tests.

[rwaffen]

uh, that's nice and simple. i like simple things 😃