Media - pre-check handling

[dustymc]

refs:

#4055 (comment)
#3950
#2986 (comment)
#2513 (comment)
and probably more

This comes up from time to time, but I don't think there's any recent discussion or sufficient documentation, and there seems to be some confusion about how this works.

Media links (http..../media/...?open) pre-check the resource by calling a HEAD request with a timeout (currently 3 seconds). If this returns a 200 status code the link works as expected, if not there's a message displayed and the user is provided a direct link.

The results of this operation are available in reports/exit links.

This is intended to do two things:

1. Log the request, including any problems with the check request
2. In the event of an unsuccessful HEAD request, alert the user that the external resource may be broken. (It may also just be refusing to talk to Arctos - perhaps that needs clarified?)

Other parts of Arctos which may involve external resources do not do this - eg Other IDs are identifiers, a broken link does not necessarily equate to the identifier being less useful.

Should Arctos somehow be doing something different?