Member Types: add member type(s) class to bp_get_member_class()

[Offereins]

Since Member Types have now a solid place in BP, let's integrate it in more locations where it could be of use.

Idea: add a member type(s) class to bp_get_member_class(), which is used when displaying members in the members loop. This could be used for visual distinction between different member types or other member-type specific logic in the loop.

Patch attached.

[slaFFik]

See #6996