[FIXED JENKINS-19081] Offer the option of downloading metadata directly from the server.
Be a bit paranoid and ensure the admin has CONFIGURE_UPDATECENTER before turning off signature checks.
@kohsuke prefers that the option to disable signature checks not even be presented unless -Dhudson.model.DownloadService.noSignatureCheck is used.
Merge branch 'master' into DownloadSettings-JENKINS-19081

Conflicts:
	core/src/main/java/hudson/model/DownloadService.java
	core/src/main/java/hudson/model/UpdateSite.java
Testing the loadJSON method directly to make sure various JSONP formats are handled.
Deleting failing test updateDirectlyWithHtml.
loadJSON does not support parsing HTML.
Nor should it need to, since UC URLs are supposed to be to the JSONP file.
The .html extension is added automatically by downloadService when postBack is supported.
Merge branch 'master' into DownloadSettings-JENKINS-19081

Conflicts:
	test/src/test/java/hudson/model/UpdateSiteTest.java
May as well use new DownloadService.loadJSON method here.
Merge branch 'master' into DownloadSettings-JENKINS-19081

Conflicts:
	test/src/test/resources/hudson/model/update-center.json.html
Merge branch 'master' into DownloadSettings-JENKINS-19081
Removed DownloadSettings.checkSignature; back to using a plain system property for this.
[FIXED JENKINS-19081] Noting merge of #1144.