Disable Special:PasswordReset when $wgEnableEmail is false

Disable Special:PasswordReset when $wgEnableEmail is false

Previously one could still navigate to the page by entering its URL directly,
and from there be able to request a password reset e-mail for any user who has
an e-mail address configured. This patch adds a check for $wgEnableEmail in the
SpecialPage code, replacing the interface with an error message when e-mail is
not enabled.

Change-Id: I42be68c6a068aafdf45623eb35bb21aa684f54f6