cmd/syncthing: Refactor out staticsServer (prev. embeddedStatic) a bit

The purpose of this operation is to separate the serving of GUI assets a
bit from the serving of the REST API. It's by no means complete. The end
goal is something like a combined server type that embeds a statics
server and an API server and wraps it in authentication and HTTPS and
stuff, plus possibly a named pipe server that only provides the API and
does not wrap in the same authentication etc.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3273

cmd/syncthing/gui_test.go

@@ -116,7 +116,7 @@ func TestAssetsDir(t *testing.T) {
 	gw.Close()
 	foo := buf.Bytes()
 
-	e := embeddedStatic{
+	e := &staticsServer{
 		theme:    "foo",
 		mut:      sync.NewRWMutex(),
 		assetDir: "testdata",