Add ClassesBeforeIds linter

Add linter which checks that classes appear before IDs in tags.

This is so that reading the tag from left to right increases in order of
specificity. Since the `%tag` portion must always be first, classes and
then IDs must follow.

Change-Id: Iebde7db6ce1678fe6a0acdcb4c2e46261a82e0fc
Reviewed-on: http://gerrit.causes.com/42339
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>