CI refactoring

This PR fixes the branch/version numbers in the Continuous integration page for the contributing documentation.

A couple of choices have been  made as part of these commits:
- remove unnecessary definitions from `omero/conf.py`
- move `split_release()` to `common/conf.py`
- hardcode the version numbers in `contributing/conf.py`. This removes the need for (2) environment variables to describe the current release and the next major release but will require PR opening for every release.
- add missing jobs to the continuous integration page.