Misc/refactor

## Ready State: Yes

Relates to https://imagex.atlassian.net/browse/LDC-417

On adding a panelizer layout it became clearly the groups functionality was far from complete. 
This replaces field_group & with field_group_group as OG recommended & exports the OG reference field base that other features should use to enable a content type as group content.

## Requirements & Dependencies

None

## Deployment Instructions

* Merge code.
* Enable module `imagex_group`
* Clear caches with `drush cache-clear all`

## Usage/Test Instructions

* Run install & ensure the groups content type contains field_group_group & going to //admin/config/group/fields shows the group content type as a group.