Move Rails/Date and Rails/TimeZone to basic cops

Using Date.today and Time.now is a common mistake which might lead to
obscure bugs. Now we're making sure we'll receive a warning when a pull
request uses these methods.

.rubocop_basic.yml

@@ -56,12 +56,18 @@ Rails/ApplicationJob:
 Rails/ApplicationRecord:
   Enabled: true
 
+Rails/Date:
+  Enabled: true
+
 Rails/HttpPositionalArguments:
   Enabled: true
 
 Rails/RelativeDateConstant:
   Enabled: true
 
+Rails/TimeZone:
+  Enabled: true
+
 RSpec/NotToNot:
   Enabled: true