Users can be added to groups twice

[johnjamesjacoby]

Experienced a strange race condition where the same user was in the same group twice. Haven't done any investigating yet, but seems likely we need to check in BP_Groups_Member::save() for the user already being in the group before we insert them.

[boonebgorges]

(In [5974]) Check to see whether a user is already a member of a group before creating a new membership in BP_Groups_Member::save(). Fixes #4113

[boonebgorges]

(In [6276]) Ensure that already-invited members can also request membership to private groups

This fixes a regression from r5974, in which an overly greedy check was put in
place to avoid duplicate group memberships. The pre-save check now only bails
when a confirmed membership is found.

Fixes #4113

[boonebgorges]

(In [6277]) Ensure that already-invited members can also request membership to private groups

This fixes a regression from r5974, in which an overly greedy check was put in
place to avoid duplicate group memberships. The pre-save check now only bails
when a confirmed membership is found.

Fixes #4113

[boonebgorges]

Ignore those commit messages. They were supposed to point to the related ticket #4495