Integrate the functionality provided by the addrelease plugin into Picard itself. One reason this was originally a plugin was because of the hacky way this worked (directly passing form parameters via query string). But the MB server's release editor seeding [1] allows to cleanly implement this.

it also allows us to fix PICARD-2202 by serving the add release form (which is necessary to do the POST call) via Picard's built-in webserver.

[1] https://musicbrainz.org/doc/Development/Release_Editor_Seeding