Introduce bbp_redirect() to wrap wp_safe_redirect() & exit()

[johnjamesjacoby]

Since all of bbPress's redirects happen theme-side, we use wp_safe_redirect() everywhere, and in 50+ locations. We also need to remember to exit() when doing so, which isn't difficult, but has caused bugs in the past.

BuddyPress uses bp_core_redirect() for this, and it has worked very well at preventing the introduction of new bugs surrounding redirection.

[johnjamesjacoby]

In 5658:

API: Introduce bbp_redirect()

• Wraps wp_safe_redirect() & exit() calls
• Prevents errors from potentially empty wp_get_referrer() results
• Replace wp_safe_redirect() usages with bbp_redirect()

Fixes #2778.

[johnjamesjacoby]

In 5659:

API: Add more context tobbp_redirect() documentation. See #2778.