Enable some Lint cops to prevent warnings

Parity with ActiveAdmin to prevent introduction of new warnings

Manually fix a `Lint/UselessAssignment` warning

Ref: activeadmin/activeadmin#8597

Author: tagliala

.rubocop.yml

@@ -86,6 +86,15 @@ Layout/TrailingWhitespace:
 Layout/TrailingEmptyLines:
   Enabled: true
 
+Lint/AmbiguousOperator:
+  Enabled: true
+
+Lint/AmbiguousRegexpLiteral:
+  Enabled: true
+
+Lint/UselessAssignment:
+  Enabled: true
+
 Packaging/BundlerSetupInTests:
   Enabled: true