Add an action to detect first login

[DJPaul]

As a result of [3691], where the ability to upload an avatar before account activation was removed from the core template, we need to add an action in that fires when the user logs in for the first time.

I believe we can do this by creating the 'last_activity' user meta on login instead of on activation.

[DJPaul]

The idea behind this new action hook would be to re-implement the avatar upload prompt, but (preferably) on first login.

[DJPaul]

I am about to commit a patch where I changed when the last_activity user meta is initially set. There was not a natural place to add an action without adding a new function, so:

Hook into the 'wp' action and check if the user is logged in and if get_user_meta( $bp->loggedin_user->id, 'last_activity' ) is false. This will be the first login.

[djpaul]

(In [3718]) Don't set last_activity on user account activation. Fixes #3003.