Read the Docs integration
Closes #282 (closed)
Generic GitLab does not need a Read the Docs integration, since Read the Docs has a Gitlab integration, but Heptapod needs one.
This integration is disabled for Git repositories (not yet available in the Web UI, but partly working on Heptapod 0.13).
Since the URL target should most of the times be under https://readthedocs.io, the TLS verification is enabled by default.
This will send a POST to readthedocs for each named branch. We may want to complement with some filtering later on.