Add GitLab Web Hook End-Point

[ghost]

Need to add an end-point to Read the Docs to received on-push events from a GitLab installation.

The GitLab end-point would be similar to Read the Docs' existing GitHub support with only minor changes needed to support the slightly different response sent by a GitLab web hook POST.

POST body sent as a result of a push event is documented here.

[ghost]

I would like to assign this to myself.

[ghost]

I just realized, there is test code for the GitHub end-point in test_post_commit_hooks.py. No Bitbucket tests though.

Should I add tests for GitLab? If so, how would I go about doing that since it seems the file is setup for only testing GitHub?

[agjohnson]

Yes, we should have tests for integrations, including the missing ones for bitbucket. I'll open a separate issue for that.