Add the concept of "system blocks"

Add the concept of "system blocks"

Blocks made for configured proxies, dnsbls, or the configured range
soft-blocks being added in I6c11a6b9 aren't real blocks stored in the
database. Let's actually flag these blocks as such and use a more
appropriate message when displaying them to the user.

Change-Id: I697e3eec2520792e98c193200c2b1c28c35bf382