Content Management via Blobs

Change the front page blurbs using blobs! Blurb blobs!

The "Non Production Banner" which used to be controlled by the EnvironmentName and set to built-in content is now entirely controlled by the Alert.md content blob (I'm going to call them blurb blobs from now on). So as part of this, we'll need to put appropriate blurb blobs on QA and Staging. Production should NOT have an Alert.md until we actually have an alert to display. Alerts are loaded via an AJAX call to the /api/v2/service-alert endpoint which returns simple HTML. As a result, you may notice that the alert slides down on first load.

Content files are ALWAYS markdown, but since markdown can contain HTML, that's OK. The web.config settings load content from App_Data\Files\Content (meaning we actually are checking in one sub-folder of App_Data). When PackageStoreType is AzureStorageBlob, we use the "content" container of the storage account.

Fixes #901