When switching between branches with different module naming schemes, the git tree is left unclean sometimes

Steps to reproduce:
1. Be on any branch. (Let's say `r/4.x`.)
2. Build Opencast
3. Switch to a branch with a different module naming scheme. (In our example this could be `develop`.)

Actual Results:
`git status` shows untracked files in `modules/{matterhorn-}admin-ui{-ng}`.

Expected Results:
The working copy should be clean.

Workaround (if any):
Just delete the untracked files or use this one git command to clean out the work tree which I always can't remember. It's no big deal really, but it bugs me. xP

The culprit in this case is the `.tmp` folder created by `usemin` during the admin UI build. This could just be placed in `target/grunt` together with all the other artifacts grunt creates. Or it could just be ignored, though I prefer solution number 1.

PR is underway.