Make it possible to prevent deprecated functions from loading

Until version 1.1, BP had a constant BP_IGNORE_DEPRECATED that kept BP from
loading the files that contain deprecated functions. This was mysteriously
removed for 1.2. Meanwhile, it was impossible to prevent deprecated functions
from being loaded.

This changeset reintroduces BP_IGNORE_DEPRECATED and uses it to decide whether
deprecated functions should be loaded. There is also a new filter
'bp_ignore_deprecated' that serves a similar purpose, but in filter form.

Fixes #5361

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7767 cdf35c40-ae34-48e0-9cc9-0c9da1808c22