Consider using CSV file as canonical list of sites

[conorsch]

Right now the CSV file is mostly useful for dev data, so developers can spin up a local instance that tracks site. In production (at https://securethe.news/), we manage the list of sites via the Wagtail admin interface. That works well enough for internal staff managing changes, but doesn't jive well with contributions to this repo.

If we had an idempotent manage.py command that loads the sites, we could merge new sites to that list, then deploy the changes directly to the prod site, without needing to futz with the clicky-clicky.

See discussion in #106 (comment) @eloquence expressed interest in implementing.