Allow the application to be parameterised so that some fields are restricted to a set of possible values

Right now, any registered user of the [SILL instance](https://code.gouv.fr/sill/) can add *any* software, even if the license is not a Free Software license.

We decided to allow this because Wikidata does not always contain accurate and up-to-date licensing information. When a non-free software is added, we simply dereference it, explaining why.

This works so far because we (the DINUM team and the SILL community) are closely monitoring new entries.

For the SILL instance and possibly other instances, it might be desirable to constrain some fields to a set of limited values.

In this case, we could prevent software addition if its license is not in the list of [accepted licenses](https://code.gouv.fr/fr/doc/licences-libres-dinum/).