Make Collection Controls Aria Accessible

[lcreid]

Make collection_checkboxes and collection_radio_buttons produce output that's more compliant with https://www.w3.org/WAI/tutorials/forms/grouping/#associating-related-controls-with-wai-aria.

Since the markup changes noticeably, this change is only enabled by setting the configuration option group_around_collections = true:

# config/initializers/bootstrap_form.rb

BootstrapForm.configure do |config|
  config.group_around_collections = true
end

This PR includes changes in the test files:

• Moved the collection_checkboxes and collection_radio_button tests to their own files.
• Test collection_checkboxes and collection_radio_button with the group_around_collections options both true and false.

Eventually, this behaviour should become the default, and later, the old behavour should disappear altogether.

Closes: #763.