Configure direct download of files to browser

Gerrit by default wraps all file downloads in zipped archive. This
can be overridden using the mimetype setting[1]. This change allows
downloading of xml, yaml, rst, html, and txt files without being
wrapped inside of a zipped archives.

[1] https://review.openstack.org/Documentation/config-gerrit.html

Change-Id: I47563de12a2bc8c8a3755fa777abf029cc4e0c28
Merge "Configure direct download of files to browser"