Add GH/Jenkins extlinks to the model documentation

Summary of changes:
- moves OMERO specific Github/Jenkins extlinks to `omero/conf.py`
- implements OME model specific Github/Jenkins extlinks in `formats/conf.py`
- fixes the source code/javadoc links under `formats/` using the newly defined extlinks.

Two important consequences:
- both sets of documentations currently use the same environment variable, namely `JENKINS_JOB`, to point at the Jenkins job. That means we need to split OMERO-docs-merge-{stable, develop} into OMERO-docs-merge-{stable, develop} and FORMATS-docs-merge-{stable, develop}. Alternately, we could use explicit environment variables, e.g. `OMERO_JENKINS_JOB`, `MODEL_JENKINS_JOB` and change the job configs. 
  /cc @hflynn, @joshmoore
- this logic allows to set the source code branch using the  `SOURCE_BRANCH` environement variable. If this PR is merged, the config of FORMATS-docs-release-stable config will have to be updated to define SOURCE_BRANCH. Ideally, we would like to use FORMATS_RELEASE envvar to set the release SOURCE_BRANCH (as in OMERO-dosc-release-stable). This implies the backporting of the 2012-06 model into bioformats.git. /cc @qidane, @rleigh-dundee