Fix unsubscribing from newsletter

Introduce authorization events

Rename authentication handler

Rename package `byceps.signals.auth` to `byceps.signals.authn`, rename signals

Rename package `byceps.events.auth` to `byceps.events.authn`

Introduce authorization domain service

Pass user object instead of ID to `authz_service.deassign_all_roles_from_user`

Pass user object instead of ID to `authz_service.deassign_role_from_user`

Pass user object instead of ID to `authz_service.assign_role_to_user`

Recompile translations

Announce newsletter (un)subscriptions

Introduce newsletter events, domain service

Introduce `SubscriptionUpdate`

Move `SubscriptionState` to `byceps.services.newsletter.models`

Pass newsletter list objects instead of IDs on newsletter (un)subscription

Verify newsletter list ID for brand before user creation

Introduce `newsletter_service.get_list`

Move `UnknownListIdError` to separate errors module

Pass user objects instead of IDs on newsletter (un)subscription

Return 401 instead of 403 from site login for invalid credentials