Validate asset configs to use https:// addresses #164

Asset-related configuration fields, such as logo_asset and favicon_asset need to be URLs.

Using a non-HTTP or HTTPS url will cause Rails to attempt to find the asset in its local asset pipeline (app/assets/images/...), which will fail, and take down the website. This also takes down the management UI, which makes fixing the error even more difficult.

Might be easiest to solve this by setting some sort of URL validator for any config field that includes _asset in the name