Update DB setup instructions for importer

[kellnerd]

Just a few documentation changes from my notes to get the importer UI working.

[kellnerd]

I've marked this as draft until I am done with testing the importer UI because I just found another missing migration... This time it was even crashing the server:

Uncaught Exception:
bookbrainz-site  | error: SELECT 
bookbrainz-site  |                        editor_id,
bookbrainz-site  |                        service,
bookbrainz-site  |                        access_token,
bookbrainz-site  |                        refresh_token,
bookbrainz-site  |                        token_expires::text,
bookbrainz-site  |               scopes
bookbrainz-site  |               FROM bookbrainz.external_service_oauth 
bookbrainz-site  |              WHERE editor_id = 1879 
bookbrainz-site  |                AND service = 'critiquebrainz' - relation "bookbrainz.external_service_oauth" does not exist

As soon as I have tested my schema change to give imports BBIDs and combine the entity and import tables, I should probably abandon the minimal test DB and start using the full DB dump again, they should be much more reliable.

[kellnerd]

Should be ready now that metabrainz/bookbrainz-site#1103 and metabrainz/bookbrainz-data-js#319 have been opened.

[MonkeyDo]

Looks fine.
I will look into automatically generating a minimal test DB at the same time as we generate the regular DB dumps, that way we can have a ready-to-use, always up-to-date minimal DB for testing.

[MonkeyDo]

Ideally we'll get rid of this mess soon and have up to date minimal DBs available somewhere