Feature/expand corpus model

[lukavdplas]

This is a partial implementation of the steps I suggest here. The major changes are:

• The database model for corpora is significantly expanded
• Corpus definitions are loaded into the database when starting the server, rather than with every /api/corpus/ request.
• When possible, interactions with corpora now rely on the database model rather than importing the python class.

The intention here is that the expanded database model can also be filled through another method (e.g. a yaml/json file or a form). As a side benefit, it also makes it easier to do validation.

This change affects the deployment and development workflow. In deployment, the loadcorpora command needs to be added to the startup script. In development, the command is included in yarn start-back. However, when you change a python corpus definition, you will need to restart the server or run loadcorpora manually to see the changes in the interface.

The PR includes a documentation file which may be a good place to start review.

[BeritJanssen]

Nice move towards administration of corpora in the admin interface. The only small request I have is renaming the functions starting with _try to something that is more explicit about conditions and handling of failure of these attempts.