Add rules related to migrations

These are rules we can't apply to existing migrations, so I'm excluding
migrations in the affected years from this check. However, we should
probably add timestamps columns and default values in non-null columns
to at least some of the tables which don't have them.

.rubocop.yml

@@ -15,9 +15,6 @@ Performance/EndWith:
 Performance/StartWith:
   Enabled: true
 
-Rails/CreateTableWithTimestamps:
-  Enabled: true
-
 Rails/EnumUniqueness:
   Enabled: true
 
@@ -39,9 +36,6 @@ Rails/HasManyOrHasOneDependent:
 Rails/InverseOf:
   Enabled: true
 
-Rails/NotNullColumn:
-  Enabled: true
-
 Rails/ReversibleMigration:
   Enabled: true
 

.rubocop_basic.yml

@@ -179,6 +179,11 @@ Rails/ApplicationJob:
 Rails/ApplicationRecord:
   Enabled: true
 
+Rails/CreateTableWithTimestamps:
+  Enabled: true
+  Exclude:
+    - "db/migrate/201[5-8]*"
+
 Rails/Date:
   Enabled: true
 
@@ -192,6 +197,11 @@ Rails/FindBy:
 Rails/HttpPositionalArguments:
   Enabled: true
 
+Rails/NotNullColumn:
+  Enabled: true
+  Exclude:
+    - "db/migrate/201[5-7]*"
+
 Rails/OutputSafety:
   Enabled: true
   Severity: warning