Working modules and Python Virtual Environments

The changes can be tested on the Test Portal

Fixed an issue with the modules, which were shown in the form, but when the job was submitted, it failed to load a module. The reason seems to be the different environment on the Portal, which somehow seems to be inherited by the job.

Also (main thing), added the option to select a Python Virtual Environment, and have it sourced in the job.

OpenComposer only looks in $HOME/venvs for virtual environments, which is also the path described on the wiki. I've also added a link to the wiki article explaining how to setup and use a virtual environment.