Remove BuddyPress's restriction spaces in user_login

BuddyPress has historically enforced a no-spaces rule on user_login during
user registration. Originally this was rooted in WPMU's own peculiar character
restrictions, and when the MU requirement was dropped, the same restrictions
were carried over to WordPress Single.

However, these restrictions have caused various problems. BP enforced the "no
spaces" rule during registration by simply swapping out spaces with hyphens and
not telling users. This caused immense confusion. Moreover, the restriction
caused problems when bypassing BP's native user registration, as when
integrating with an external authentication service; these external usernames
*can* sometimes have spaces, and certain areas of BuddyPress were not equipped
to deal with them.

This changeset removes the no-spaces restriction from BuddyPress, and hands
off user_login validation to WordPress Multisite when possible (meaning that on
MS, spaces will still not be allowed during native registration). It also
makes the necessary adjustments throughout BuddyPress to ensure that spaces
in user_login will not break related functionality. On a normal setup, BP (and
WP) only use user_login for authentication, but several changes were necessary
to account for "username compatibility mode", where the user_login is displayed
publicly:

- Refactor the way that activity @-mentions work in username compatibility
  mode. We now have functions for converting user IDs to "mentionname" (and vice
  versa) which will produce @-mention-safe versions of user_nicename or
  user_login, as appropriate.
- Use proper URL encoding when building and parsing URLs that contain usernames
  when compatibility mode is enabled.
- Fix private messaging autocomplete to work with spaces.

See #4622

Fixes #5185

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7570 cdf35c40-ae34-48e0-9cc9-0c9da1808c22