Add and apply Rails/WhereRange rubocop rule

This rule was added in rubocop-rails 2.25.0. Applying it allows us to
simplify the code a little bit. For example, now there's no need to
specify the `proposals` table in proposal scopes, which was actually
causing a bug in the `Legislation::Proposal` model, which was using the
`proposals` table instead of the `legislation_proposals` table (but,
since we don't use this scope, it didn't affect the application).

.rubocop.yml

@@ -517,6 +517,9 @@ Rails/WhereNot:
 Rails/WhereNotWithMultipleConditions:
   Enabled: true
 
+Rails/WhereRange:
+  Enabled: true
+
 RSpec/AroundBlock:
   Enabled: true