Add validator that checks for blocked content types

Project owner can now configure blocked content types. Commits
containing files with blocked content types will be rejected.

To detect content type, we use Apache Tika [1]. Apache Tika was
introduced in change Ia2529f187b8362ad2f21bb5e9bd9d0f88eb775b5 .

[1] https://tika.apache.org/

Change-Id: I2d2f67642d6ee43d311a582890bd23bb05233142