Refactor switching login type code into app layer and add v4 endpoint (#6000)

* Refactor switching login type code into app layer and add v4 endpoint

* Fix unit test

model/switch_request_test.go

@@ -0,0 +1,19 @@
+// Copyright (c) 2017 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSwitchRequestJson(t *testing.T) {
+	o := SwitchRequest{Email: NewId(), Password: NewId()}
+	json := o.ToJson()
+	ro := SwitchRequestFromJson(strings.NewReader(json))
+
+	if o.Email != ro.Email {
+		t.Fatal("Emails do not match")
+	}
+}