Data coherence should be improved after the deletion of some resources

[mmo]

Bug description:

The deletion of some resources cause some operations to fail, because "dead links" are created in other resources, resulting in data coherence problems. (See "Expected behavior" below for the actual cases.)

Expected behavior:

• deleted documents should be unlinked from their deposits
• deleted users should be unlinked from their deposits
• deleted subdivisions should be unlinked from their users and deposits

Notes:

• there may be other cases - to be checked
• in some cases, instead of unlinking resources, it is perhaps better to delete them (ex. documents/deposits)
• in some cases, instead of unlinking or deleting associated resources, the deletion operation should be forbidden upfront (ex. forbid the deletion of users with pending deposits, or the deletion of subdivisions with attached users or pending deposits)