feature-request: add healthchecks for mirror servers

[ericswpark]

Description

Mirror servers may go down from time to time. If shipper detects that a given mirror server is no longer healthy (by sending a HEAD request to a random downloadable build), it does the following:

1. Checks to see if the main server is enabled (if not, stop here)
2. Turn off "downloadable" for the affected server
3. Send an email to the administrators

Additional context

Alternate approach: show un-"downloadable" mirrors in the mirror list, but show an "offline" indicator. That way, we can immediately switch back when the mirror comes back up.

[ericswpark]

I like the client checks the health status approach a bit better. Have the client UI send a HEAD request, and if the mirror server is not available then show a crossed-out heart indicator showing that it is down.