HAML files (ending with `.html.haml.rb`) added (manually) to
`.rubocop_todo/**/*.yml` now are retained and not removed.

This is useful because `haml-lint` (which use RuboCop rules) cannot
exclude HAML files per RuboCop. So, we tend to disable whole cop rules in
`haml-lint.yml`.

With this change we can now start enabling "temporarily" disabled cop
rules in add HAML files as TODOs.

Using the former has been deprecated:

  $ rubocop --auto-correct
  --auto-correct is deprecated; use --autocorrect instead.

Sometimes we enable new cop rules without any offenses and put this rule
in "grace period". This is done in the TODO config in
`.rubocop_todo/**/*.yml` because it's a temporary configuration which
should be removed short after.

However, when regenerating TODOs for cops without any offenses we
discarded to generate them.

This commit fixes this issue and generates a cop TODO config if:
* Cop was previously disabled due to too many offenses
* Cop is in grace period
* Cop excludes files

Disallow the case where a cop is disabled _and_ has a grace period.

It will be used by new rubocop rake task soon.