Show ZERO info screen on banner click

!!!EXPERIMENTAL!!!
Visual design, architecture, and text need thorough review.

Implements a popup screen shown when the user clicks on the banner.
The popup describes what the Zero program is, what other languages (sites?)
have also been zero-rated by the current carrier, HTTPS, proxies?, etc.

The page also shows a link to the carrier's own Zero info page.

Change-Id: I7b54d7df43328da09936ef582bbe94cab05df786
Merge "Show ZERO info screen on banner click"