Show banner when processor is loading data for the first time

The cold start on full set of projects is quite slow. Usually it
takes about 3 hours to load all commits, reviews, blueprints and emails,
and up to 8 hours to load all members data. During this period users
need to be notified that the data is incomplete.

Closes bug 1322177

Change-Id: I04432e29e3e7bbfad1a61dc463bd9193e27dde9e