Trunk 3415 Add ability to update css through webapp

3 commits comes from https://github.com/ehsdev/openmrs-module-custombranding repo, without them original custombranding repo didn't build
Remember that controllers can serve multiple requests concurrently, thus class fields can be accessed and modified concurrently by multiple threads. It is not correct to store in them the state for a single request. I see that you use at least currentFile that way and possibly others.