Schema for campaign throttling

Change-Id: Ib82117a1b53eeca3e2aeeeb8e4676e61291146fa
(FR #1048) Campaign throttle knob

This patch contains the support and UI for the feature, but does not affect
allocations.

Campaign throttle allows CentralNotice admins to limit the traffic going to
their banners to a percentage of the total potential maximum allocation.
Leftover allocations are rolled over to campaigns at the next lower priority
levels.

Change-Id: I04d81df646c62580b5337b90f32db04c2ca37f59
Merge changes I04d81df6,Ib82117a1

* changes:
  (FR #1048) Campaign throttle knob
  Schema for campaign throttling