dashboard: Add a footer with useful information

The footer contains a link to the gitlab repo, and also displays the git commit id of the running instance. This is useful to ensure we are running the expected version in production and provide more context in bug reports.

The GIT_COMMIT_ID variable is made available to all templates by using a custom context processor.

This also works with our docker image, since the image contains the 'git' binary which we are using to obtain commit information.

Signed-off-by: Alexandros Frantzis alexandros.frantzis@collabora.com