FEATURE: introduce dedicated storage and DB constraints for anon users

Previously we used custom fields to denote a user was anonymous, this was
risky in that custom fields are prone to race conditions and are not
properly dedicated, missing constraints and so on.

The new table `anonymous_users` is properly protected. There is only one
possible shadow account per user, which is enforced using a constraint.

Every anonymous user will have a unique row in the new table.

spec/services/anonymous_shadow_creator_spec.rb

@@ -34,6 +34,9 @@ describe AnonymousShadowCreator do
       expect(shadow.id).to eq(shadow2.id)
       create_post(user: shadow)
 
+      user.reload
+      shadow.reload
+
       freeze_time 4.minutes.from_now
       shadow3 = AnonymousShadowCreator.get(user)
 
@@ -56,6 +59,7 @@ describe AnonymousShadowCreator do
       expect(shadow.created_at).not_to eq(user.created_at)
 
       p = create_post
+
       expect(Guardian.new(shadow).post_can_act?(p, :like)).to eq(false)
       expect(Guardian.new(user).post_can_act?(p, :like)).to eq(true)