[PP] Merge og_ui into the main OG module

[pfrenssen]

It is no longer making sense to keep these two modules separated. We can safely assume that 80% of the sites that use Organic Groups will need at least some part of the UI in the day-to-day operation of the site.

By merging the two we can meet the needs of most users and avoid the complexity of maintaining two different modules.

[MPParsley]

Postponed on #243.

Note that currently the OgRole entity is defined in 'og' but it contains links to routes defined by 'og_ui':

 *   links = {
 *     "add-form" = "/admin/config/group/roles/{entity_type_id}/{bundle_id}/add",
 *     "edit-form" = "/admin/config/group/role/{og_role}/edit",
 *     "canonical" = "/admin/config/group/role/{og_role}/edit",
 *     "delete-form" = "/admin/config/group/role/{og_role}/delete",
 *   },

This won't work when reinstalling the site from config. Merging og_ui with og would solve this.