Add Rubocop exclusions for Rolify stuff

Since adding Rolify, we are triggering two significant Rubocop offenses:

- `has_and_belongs_to_many` is frowned upon in favor of
  `has_many :through`, but Rolify still uses the former. There have been
  many attempts to resolve this (see the Rolify issues and PRs) over the
  past decade, but none have fully landed. It’s beyond us to try and
  wrestle this into existence, so we’re going to make an exception and
  move on.
- The HABTM join table has no timestamps, which is also frowned upon. We
  genuinely don’t care, however, and so are making an exception.

Issue #299

.rubocop.yml

@@ -19,7 +19,7 @@ AllCops:
 # Join tables don't really need timestamps
 Rails/CreateTableWithTimestamps:
   Exclude:
-    # - 'db/migrate/20180128231930_create_organizations_and_events.rb'
+    - db/migrate/20220919194501_rolify_create_roles.rb
 
 # Rails generates this file
 Style/BlockComments:
@@ -64,6 +64,10 @@ Rails/HasManyOrHasOneDependent:
 
 Rails/HasAndBelongsToMany:
   Enabled: true
+  Exclude:
+    # Rolify uses HABTM. Despite a decade of the community attempting to implement
+    # `has_many: through`, it still struggles mightily with it. Let's make an exception.
+    - app/models/role.rb
 
 Style/NumericPredicate:
   Enabled: true