Send email from $wgPasswordSender instead of user's email

This is to stop emails going to spam if an SPF record is set.

This is required for the extension to work on wikis hosted
by sites with strict sender domain policies (like Dreamhost).

See: https://www.mediawiki.org/wiki/Thread:Extension_talk:InviteSignup/Sending_mails_from_users%27_addresses_is_not_a_great_idea

Change-Id: If495b325c228f7257117c460ff9e107aa0cc7082