Signups: move the activation link's key from the query param into the URL proper.

Some email hosts strip the key from the activation link, or mangle it, or prevent it being easily copyable inside their web interfaces. Some web servers/configurations, can also be commonly configured to strip query parameters from the majority of requests, so hopefully this change will alleviate these problems.

Compatibility is maintained for the older query param-style activation links.

Fixes #5831, props r-a-y.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9109 cdf35c40-ae34-48e0-9cc9-0c9da1808c22