Issue 37: UI to upload tiles without deploying

Fixes #37:

This PR includes backend and front-end code that makes Tiles deployment safer, and adds another feature: the ability to add tiles to the Data Warehouse without deploying.

It is a stepping stone towards the incoming scheduling feature of tiles deployment.

The UI features include an additional step to deploy tiles: a checkbox needs to be checked before the data is deployed to Firefox users.

Fixes #40:  (This should've been a separate PR... sorry)

Another feature in this PR is the concept of Channels.
This allows for the deployment of Tiles to channels other than Desktop, e.g. Android.
I might be missing something, but I don't see where the migration (for Redshift) has a 'created_at' column