Support server, deprecate backend in plugin manifest (#9127)

* Support `server`, deprecate `backend` in plugin manifest

This lets us converge on the use of the term `server` everywhere instead
of sometimes `backend` and sometimes `server`. We're still using
`webapp` and will eventually support `mobile` as well.

The plan is actually to rip out these deprecations as part of releasing
5.2, but I want to coordinate the extra additional breakage at the same
time, so for now this is a backwards compatible change.

* fix failing tests

model/plugins_response_test.go

@@ -10,7 +10,7 @@ import (
 func TestPluginsResponseJson(t *testing.T) {
 	manifest := &Manifest{
 		Id: "theid",
-		Backend: &ManifestBackend{
+		Server: &ManifestServer{
 			Executable: "theexecutable",
 		},
 		Webapp: &ManifestWebapp{