Get rid of discr property and merge it with type

[vivienlacourba]

type property is needed on:

• groups (various types of groups wg / ig / bg / cg)
• affiliations (to differentiate between affiliation and organization)
• Not needed elsewhere as it is obvious which type of object we are getting based on the endpoint

[jean-gui]

discr is present because it would be necessary for deserialization (in case of a POST or PUT). It is possible to rename it, but then you also have to explicitly add all the value<->class pairs, which will be a pain to pain.

I think it's better to let it be.

[vivienlacourba]

I agree.