refactor file controller to use Mojolicious::Static

While I was at it, I also changed other things:

* the thumbnail is always scaled to 120x120 - and os-autoinst will prepare one
in the future.
* you can only download ISOs for tests you can access (right now everyone can)
* there is only one test_file function, both serving test files and uploaded log files
Merge pull request #13 from coolo/static_file

refactor file controller to use Mojolicious::Static