Deprecate $wgPasswordSenderName

The sender name for system mailings can now be configured
locally by modifying the system message "emailsender". The
new default sender name is simply "{{SITENAME}}". Added to
release notes.

Also modify UserMailer to strip CR/LF linebreaks from header
values to prevent mail header injection now that the sender
name can be modified.

Bug: 32770
Change-Id: Ibfd28cd181365c8c0b5f3e8ffe8f5de8c89844a3
Merge "Deprecate $wgPasswordSenderName"