Migration outbox filtering

[aaronjae22]

/api/actors/<pk>/migration/outbox/ now serves only the Create activities that wrap a Note and every migration collection is identified by its own URL. /api/actors/<pk>/outbox/ is unchanged.

We had the migration outbox resolved to the same view as the outbox so a destination walking the collections the Actor advertises received:

• every Like twice since we would serve a Like activity in the migration outbox, and its object in liked
• every Follow twice: a Follow activity there, and the followed actor in migration/following
• the Actor itself which is wrapped in the Create that announces account creation

Currently

/api/actors/1/migration/outbox/     id …/outbox/      totalItems 43
  types Create ×31, Like ×6, Follow ×6   Create objects: Note ×30, Person ×1
/api/actors/1/migration/content/    id …/content/
/api/actors/1/migration/following/  id …/following/
/api/actors/1/migration/blocked/    id …/blocked/

objects also served elsewhere:  liked 6   migration/following 6

After this PR

/api/actors/1/migration/outbox/     id …/migration/outbox/     totalItems 30
  types Create ×30                       Create objects: Note ×30
/api/actors/1/migration/content/    id …/migration/content/
/api/actors/1/migration/following/  id …/migration/following/
/api/actors/1/migration/blocked/    id …/migration/blocked/

objects also served elsewhere:  liked 0   migration/following 0

/outbox/ is identical before and after: 43 items bound, 19 anonymous. Anonymous migration/outbox/ goes from 19 to 10, the public Creates of a Note.

• LOLA §6.2 (SHOULD): "This collection SHOULD contain a filtered view of all the actor's activities that the source server deems eligible for migration."
• LOLA §6.6.1 (MAY): "The source server MAY omit these types of activity when providing a migration outbox." (Like, Follow, Block)
• LOLA §6.6: "The Actor object will be consulted during account migration, but not copied."
• ActivityPub §3.1 (MUST): unique global identifiers