Add servlet to serve images

The plugin originally relied on changes in Gerrit core to serve the
images, but these changes were never merged [1]. Implement an own
servlet to make images browsable.

[1] https://gerrit-review.googlesource.com/53918

Change-Id: I9af6f9bc47ad462fd0323ea79ce62560812b63ac
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>