[WIP] API
Adds API endpoints:
- action=readinglists
- meta=readinglists
- list=readinglistentries
- list=readinglistorder
TODO:
- prevent adding/removing entries from deleted lists?
- review DB index coverage
- use non-offset-based continuation where possible
- T164990#3460750
Longer term TODO: how will the REST API use tokens?
Bug: T168980
Bug: T168988
Change-Id: I3615581fb06c599dea50877d3678d1ba3e82bb20