Display a "resend email" link when an unactivated user tries to log in.

An unactivated user is a person who registers a new account, but has
not activated their account via email yet.

Previously, if an unactivated user tried to login, there was no option
for the user to resend the activation email.  This commit adds this
option.

Props r-a-y, imath

Fixes #4676

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8170 cdf35c40-ae34-48e0-9cc9-0c9da1808c22