catalog-backend: add support for static location config + bootstrap location

[Rugvip]

Fixes #1833

Tried 3 different implementations of this, this is the least ugly one ._.

So this adds a processor that reads a 'bootstrap' location, of which there always exists one, and it can be used to bootstrap other work.

I tried implementing this as a decorator for the LocationCatalog, as you can do more validation of that locations are allowed to be added with that. But that falls apart because the locations aren't added to the database, and entities has a foreign key constraint on the location id.

Another option was to do something similar to this, but use a statically added location entity instead. This felt slightly more clean though.

Overall I ran into a bunch of issues because of the existence of LocationIds, and would want to explore if we could completely promote all kinds of locations to entities, and then also get rid of the IDs.

I also noticed the slightly awkward fact that it's the registered location in the database that shows up as the managed-by location, even when emitting a location entity that is then read to collect more entities. It's again due to the location id being a requirement, and emitted locations don't receive any ID.

[andrewthauer]

@Rugvip - There might be an issue with this migration and postgres. I started seeing this error:

migration file "20200809202832_add_bootstrap_location.js" failed
migration failed with error: insert into "locations" ("id", "target", "type") values ($1, $2, $3) - invalid input syntax for type uuid: "bootstrap"

[Rugvip]

@andrewthauer yep, fixed by #1935