Add information about Ghostery Browser on AMO

[chrmod]

For some users it may be confusing they are installing addons from addons.mozilla.org. To make it easier to understand we can add a banner on top of AMO to inform users what is happening.

This setup give us an opportunity to explain to the users few things around addons - like the fact they should not use multiple adblockers.

[chrmod]

Another use case would be to recommend addons - related https://github.com/ghostery/user-agent-desktop/issues/391