bp_has_custom_signup_page() & bp_has_custom_activation_page() are always true

[slaFFik]

These 2 functions always return true, and thus we can't use default wp-signup.php or /wp-login.php?action=register.
Reason: they check that these files exist:

array(
    'registration/activate.php',
    'members/activate.php',
    'activate.php'
)

But they always exist in bp-templates/bp-legacy/buddypress.

[boonebgorges]

This probably dates from [6448].

Checking for a template seems a pretty indirect way to see whether to load BP's registration page (rather than WP's) - it just happened to work previously. Should we check for the presence of bp_pages['register'] instead?