Allow `bp_groups_user_can_send_invites()` to accept a user ID

[dcavins]

As it functions now, bp_groups_user_can_send_invites() only tests against the logged-in user. It would be useful in some instances (like when an admin is admin-ing a group) to be able to pass it a user_id to check specifically.

I made this change in #6025 as a necessary change for error-checking in groups_send_invites(), but realize that this may be easier to deal with in a smaller chunk.

[dcavins]

Add $user_id parameter to bp_groups_user_can_send_invites()

[r-a-y]

In 9293:

Add $user_id parameter to bp_groups_user_can_send_invites().

This allows developers to check if a specific user can send group invites
rather than just do checks against the logged-in user.

This commit also:

• Updates the PHPDoc for the function and the filter hook.
• Adds unit tests.

Props dcavins.

Fixes #6031.

[dcavins]

Adds nonmember case to test cases

[dcavins]

Hi r-a-y-

Thanks very much for making improvements to the patch and committing it. Upon reading over the change set, I realized that I had missed a basic test case in the unit tests: non-members.

I've added a patch that covers this case and also tweaks the "current group" test.

Thanks again for sponsoring this patch.

[r-a-y]

In 9300:

Tests: Add tests for non-members for bp_groups_user_can_send_invites()

Props dcavins.

See #6031.