Better fallback support on Activity Admin for items with an unregistered type

The 'Type' dropdown on the activity edit screen is populated with types that
have been registered with bp_activity_set_action(). However, many legacy
plugins do not register their activity types correctly. This adds support
for editing these types of activity items.

Fixes #4786

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8085 cdf35c40-ae34-48e0-9cc9-0c9da1808c22