Virtualjob extlink

This PR should fix the link to the latest succesful `.ova` file in the virtual appliance documentation.

At the current HEAD of this branch, I have
```
sbesson:sphinx sebastien$ git grep -E "\-trunk"
README.rst:By default, the OMERO job is set to ``OMERO-trunk``. To specify a different 
conf.py:    jenkins_job = 'OMERO-trunk'
developers/Cpp.txt:- :jenkins:`Windows Server 2003 (32 bit) <job/OMERO-trunk-components/component=cpp,label=x86-windows/>`
developers/Cpp.txt:- :jenkins:`Linux <job/OMERO-trunk-components/component=cpp,label=linux/>`
developers/Cpp.txt:- :jenkins:`MacOSX <job/OMERO-trunk-components/component=cpp,label=macosx/>`
developers/continuous-integration.txt:          * :term:`OMERO-trunk`
developers/continuous-integration.txt:          * :term:`BIOFORMATS-trunk`
developers/continuous-integration.txt:  :jenkinsjob:`BIOFORMATS-trunk`
developers/continuous-integration.txt:  :jenkinsjob:`OMERO-trunk`
developers/release-process.txt:   -  See: :jenkins:`job/OMERO-trunk-components`.
developers/release-process.txt:-  Rename "\*-trunk" jenkins jobs to "\*-<RELEASE\_NAME>"
```

All the `*-trunk` links present in the developers/continuous-integration.txt page are expected. The only leftovers are the `OMERO-trunk-components` links. Once this job is green again and copied on to the stable branch, I will implement the same strategy as the one in this PR and create a `componentsjob` alias.

/cc @joshmoore, @hflynn