Use bp_locate_template to find best template to use

I noticed that my custom template file was being ignored after reorganizing my theme in the post-BP1.8 format.

Commit note: bp_locate_template() searches old template structure (/groups/, etc, at root of theme) and new style (/groups/, etc, in /buddypress/ or /community/ folder at root of theme). Does require BP 1.7+, though.