Generate avatars with transparent background

Generate avatars with transparent background (for .png images) or with white background (for others).
only the host part of an email address is case insensitive
Add ScreenedUrl. Rename BlockedEmail to ScreenedEmail.
Merge pull request #1330 from ZogStriP/emails-can-be-mixed-case

only the host part of an email address is case insensitive
Finish renaming blocked emails to screen emails
Merge pull request #1328 from abbat/patch-1

Generate avatars with transparent background