Set Celery serialization to JSON only
Revert "Issue #56, #8, now that metadata types are automatically added to a document upon type change, there is no more need for the document metadata add and remove code, view, API"

This reverts commit c390fc21d765f436d32e25e9276b4dd1a60e4279.

Re adding this feature, only required metadata should be automatically added, optional metadata would be added by the user
Remove unused template
Change the way metadata types are associated to a document type, add required boolean field to the association
Remove success messages from the assign_remove view, allow passing null lists titles
Allow form with subforms to have a main title
Remove the views associated to the link they are unnecesary
Update metadata admin
Add TODO reminder
Add manager class to handle the metadata types of a document type easily
Add view to add or remove required metadata types to a document type, simplify existing view to add remove optional metadata types
Remove remarked code
Change the name of the variable used as the main title for multiform template
Show required and optional metadata types on upload wizard
Automatically add required metadata types only when creating a new document
Add validation to block removal of required metadata types from a document
Enable required metadata types in the metadata selection form
Show which document metadata are required, show validation error when trying to remove a required metadata type from a document