Add tests for nested Repeater/MLRepeater usage

[jimcottrell]

This shows a couple of issues with repeaters that should be addressed by octobercms/october#3729 and rainlab/translate-plugin#409.

A couple of repeater groups have been added to Countries > Content. The one named Recurse allows for arbitrary levels of nesting. Currently, two levels of nesting can be added, but attempts to save the form throw an error. Attempting to add another level of nesting will similarly fail.

The Tag list group adds a taglist widget in string mode. Because repeaters are currently returning the raw form data via getSaveValue, a child taglist will never have a chance to process the save data. After adding tags to a taglist and saving, an exception will be thrown on reloading the page, as the saved value is the raw array rather than the expected string.

Both Countries > Content and Countries > States conditionally support RainLab.Translate so that MLRepeater functionality can also be verified with group and non-group usage.

[LukeTowers]

This needs to be re-evaluated in lite of octobercms/october#4234 and rainlab/translate-plugin#466