Strip devbranch out of omero/conf.py
Move split_release() function to common/conf.py
Refactor stable branch and next point/major release definition

Also improve the versioning paragraph by referring to semver.org
Add release numbers to table headings and add the Ice 3.5 jobs
Add contributing docs jobs to the CI documentation page
Harcode version numbers in conf.py
Add small general section about the continous integration server
Remove continuous integration workflow diagram
Minor fixes
Move get_previous_version logic to common/conf.py

Migrate common code to determine pervious series from current release number
to common/conf.py. This logic will be refactored to using git tags eventually.
Use directly this function in contributing/conf.py allowing to pass the number
of the next major release only to determine the stable development branch.
Do not propagate the unused major_release variable
Branch fixes
Merge pull request #573 from sbesson/ci_refactoring

Continous integration fixes