Fix <#28>

Because SidebarLinkPlugin nowadays extends GlobalConfiguration
rather than Plugin, Jenkins maps ${rootURL}/plugin/sidebar-link/*
only to static resources and not to Jelly views like startUpload.jelly.
Form validation generates URLs that use "descriptorByName",
so let's use that for the upload iframe as well.

(Descriptor.getDescriptorUrl would return that string,
but the risk of bugs seems lower if I just hardcode it.)

layout.jelly will add data-crumb-header and data-crumb-value
attributes to the head element, and include behavior.js,
which should then add the anti-CSRF crumb as a hidden field
to the HTML form.

Fix JENKINS-55710

action="upload" in startUpload.jelly attempted to call the
SidebarLinkPlugin.doUpload method, but that was renamed to
doUploadLinkImage in commit 3ef9756
during the CasC work, so we need to use action="uploadLinkImage".