Make join methods for groups re-usable

Join methods can be used for other entity types when using the new plugin type Join and the following new hooks:

1. hook_social_group_join_info_alter - alter the list of join plugin definitions.
2. hook_social_group_join_method_info - define join methods.
3. hook_social_group_join_method_info_alter - alter the list of join method definitions.
4. hook_social_group_join_method_usage - define entity type bundles that support join methods.
5. hook_social_group_join_method_usage_alter - alter the list of relations between entity type bundles and join methods.

The hook hook_social_group_request_alter is marked as deprecated and the 3rd hook from the list above should be used instead.