Update patch set 14

Patch Set 14:

@jcrespo this is the (hopefully) final version of the DB schema from the reading list RfC (T164990), plus all the DB queries, in case you'd like to review (much appreciated!). I plan to do a round of query optimization once the extension is in production and not blocking the progress of other teams (it won't get much load for a year or so), that's tracked in T171913.

Patch-set: 14