Add tags to accordius under new REST API

[JD-P]

Accordius should be able to serve the client tags using the Django Rest Framework as we begin migrating away from GraphQL.

The tags implementation based on the model in lw2/models.py should conform to these specs:

Note:
This is all in addition to the basic features of any sane tagging system; Pinboard or lobste.rs is a decent starting point.
Note 2: These desiderata are not necessarily all compatible; some may be contradictory.

• Tags may be harvested from the content itself, via special markup, or via an out-of-band input method
• Tags should be stored as metadata, separately from the content
• Any chunk of content should be taggable (post; comment; media asset)
• Optionally (to the server operator), users should be able to suggest tags
• User-suggested / crowdsourced tags should be stored separately from author-applied tags
  Whether crowdsourced and author-applied tags are displayed separately should be up to the operator of the { server | forum }
• Optionally (to the server operator), a set of pre-supplied suggested tags should exist
  Whether users are limited to the set of pre-supplied tags should be up to the operator of the { server | forum }
• Tag names should be case-insensitive but case-preserving

(Source)

[JD-P]

Progress: REST framework has been added and there is now a /tags/ endpoint, however certain things still don't work, including:

• No separate model for default/'official' tags
• No separate model for suggesting tags
• No options for how tags are 'displayed' server side
• Tag names are case preserving but are not searched case insensitive yet

[JD-P]

Tags are now shipped in the WL 2.0 interface, but there's no search and the test suite doesn't cover all the cases I'd like yet.