Refactor ComplementaryDocumentType in two separate models

[olivierdalang]

Currently, the ComplementaryDocumentType model mixes both DocumentType categories, and DocumentTypes themselves. This is not very clean as it mixes two different logic in the same model. It also forces document types to be specific to a WOT (so we can't use general types such as "plan", "photo", etc.).

Instead, we should create a distinct DocumentTypeCategory model for the parent, and have the relation to the WOT directly on the child.

(hope it's clear)

[rbovard]

@AlexandreJunod is it fixed via #642?

[AlexandreJunod]

@rbovard The user interface has been improved by creating separate views, but the model has not been modified