Use functions instead of $bp global when referencing user data

[johnjamesjacoby]

We still directly reference $bp->loggedin_user->domain and $bp->displayed_user->domain in many places, when we should be using bp_loggedin_user_domain() and bp_displayed_user_domain(), respectively.

Same goes for many of the other userdata related references.

These should be switched over to their functions.

[johnjamesjacoby]

(In [5704]) Replace direct references to $bp user domains with their functional equivalents. See #3994.

[johnjamesjacoby]

(In [5705]) Replace direct references to $bp user fullname and user_login with their functional equivalents, and kill off some $bp global usage in the process. See #3994, #3989.

[johnjamesjacoby]

Was actually a dupe of #3989. Closing as fixed.