app store: add HTTP mirror support

[dr-frmr]

Problem

Current app distribution is purely node<>node, which is cool, but a lot of developers like to host source code at a public URL.

Solution

Add support in the distro's app store to GET from http URLs in app metadata mirrors field.

Testing

(not done yet)

1. publish an app with an HTTP/HTTPS URL in its mirrors array
2. try to download said app, selecting that URL as mirror
3. make sure HTTP and HTTPS both work