[feature] Implement explicit domain allows + allowlist federation mode (#2200)

* love like winter! wohoah, wohoah

* domain allow side effects

* tests! logging! unallow!

* document federation modes

* linty linterson

* test

* further adventures in documentation

* finish up domain block documentation (i think)

* change wording a wee little bit

* docs, example

* consolidate shared domainPermission code

* call mode once

* fetch federation mode within domain blocked func

* read domain perm import in streaming manner

* don't use pointer to slice for domain perms

* don't bother copying blocks + allows before deleting

* admonish!

* change wording just a scooch

* update docs

mkdocs.yml

@@ -102,6 +102,8 @@ nav:
 
   - "Admin":
       - "admin/settings.md"
+      - "admin/federation_modes.md"
+      - "admin/domain_blocks.md"
       - "admin/cli.md"
       - "admin/backup_and_restore.md"
   - "Federation":