Upstream installation requirements

Had a problem with the latest version and checked https://github.com/gitlabhq/gitlabhq/blob/master/doc/install/installation.md and https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/upgrading_from_source.md to find the reason. The problem turned out to be my fault but I decided to incorporate the installation requirements stated by these two upstream documents into my fork of the gitlab overlay.

Besides updating some versions of required packages I made the following changes to the gitlabhq ebuild:

• added a favicon USE flag that pulls in GraphicsMagick which is needed for the Custom Favicon to work
• added the [pcre,pcre-jit] USE flags to the required dev-vcs/git because "Git is required to be compiled with libpcre2"
• added redis as second group to the enewuser command that creates the git user
• added some elog-messages noting the two required PostgreSQL extensions: pg_trgm and btree_gist

In the gitlab-workhorse ebuild I added a dependency for media-libs/exiftool which workhorse needs to remove EXIF data from uploaded images. Without this image uploads fail with an error that exiftool was'nt found.

Note that I changed the reponame in my fork to gitlab-test in order to be able to use this overlay in parallel to the original gitlab overlay on my test machine.