Revert "DefaultSettings.php: Remove $wgPasswordSenderName"

It seems that this variable is used by a whole
lot of extensions, so to remove it now would be
to break these other extensions.

This reverts commit acda2cc90aea519f41c8ae55bc4d05f02072df84.

Change-Id: I2bd3078508c0608ff58bae4c9ec456e57c617da8
Merge "Revert "DefaultSettings.php: Remove $wgPasswordSenderName""