More app code migration (#5170)

* Migrate admin functions into app package

* More user function refactoring

* Move post functions into app package

app/app.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"io/ioutil"
+	"net/http"
+)
+
+func CloseBody(r *http.Response) {
+	if r.Body != nil {
+		ioutil.ReadAll(r.Body)
+		r.Body.Close()
+	}
+}