Create tables for storing metadata about targets

[hellais]

It would be useful to have tables which can be efficiently joined against the obs_* tables that store metadata related to targets, such as category codes and resource names.

This is also important for the thematic censorship pages.

Should be done, or at least thought of, as part of #88

[hellais]

As an initial MVP for this we have something very basic which is just a URL type table that points to the citizenlab/test-lists repo.