Add Subcategory

Taking these ( https://gitlab.cern.ch/push-notifications/notifications-design/-/issues/19 ) design decisions (see "Consensus" section), restructure Categories/Tags.

Category has OneToMany to Category.

Restructure models models
Enforce new channels creation/update rules
Tweak findAllChannels filtering to accommodate changes

Edited Nov 25, 2021 by Jose Semedo