Moved Preferences::trySetUserEmail() to User::setEmailWithConfirmation()

Moved Preferences::trySetUserEmail() to User::setEmailWithConfirmation()

* Much more easier to find it in the User class than in Preferences and it's general enough to be in that class.
* Rewrote the function for better readbility
* It now always return a Status object so that it's easier to interpret its result.
* Update the only caller in core (in Special:ChangeEmail) and moved the PrefsEmailAdit hook there

Change-Id: I55939bb5295e73594c3fdf7287dddbc16a233ce4