Make og_user_roles_grant_roles more re-usable

og_user_roles_grant_roles static caches roles based on the original group and user passed in, so if user changes behind scenes or want to use it on a different user or group (other than the current) it doesn't work.

Patch does
1) Moves determining group context into the function and saves current group there for later so doesn't have to be redetermined (unless reset).
2) Caches roles based on user and rid so can be pass any account/group
3) moved og_get_group_context in front of og_user_roles_determine_context as og_get_group_context requires a lot less work so for modules using that, it should provide a performance boost.
4) Moved around some of the quering stuff as it was just strange. It was acting like it could have multiple group ids, but would never provide more than 1 group ids, so was needlessly complicated.
5) Saves the current roles in a site_roles variable so modules can see what the original roles were.

I wanted to use og_user_roles_get_roles_by_group instead of a query there, but that doesn't return the name so didn't want to needlessly slow that query and change what it returns.