BP_Group_Extension should automatically handle redirects after the 'edit' screen save routine

After handling POSTed settings save on the 'edit' view of a BP_Group_Extension
plugin, plugins were previously required to handle their own redirects
(primarily to avoid "are you sure you want to resubmit the data?" browser
prompts when reloading the page). This created a disparity between the 'edit'
screen and the 'create' and 'admin' screens, the latter two of which handled
their own redirect logic.

This changeset performs the necessary redirect after calling the edit screen
save callback. There is some logic in place to ensure that no double redirects
take place.

Fixes #5557

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8934 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Improvements to bp_blogs_total_blogs_for_user()

- Don't bother to check the cache or perform DB queries for users who are not
  logged in
- Do a strict type check when checking the cache, to ensure that a cached value
  of 0 does not result in an unnecessary database hit

Fixes #5815

Props wpdennis, r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8935 cdf35c40-ae34-48e0-9cc9-0c9da1808c22