Add render template hooks for links and images

This commit also

* revises the change detection for templates used by content files in server mode.
* Adds a Page.RenderString method

Fixes #6545
Fixes #4663
Closes #6043

identity/identity_test.go

@@ -0,0 +1,42 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package identity
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestIdentityManager(t *testing.T) {
+	c := qt.New(t)
+
+	id1 := testIdentity{name: "id1"}
+	im := NewManager(id1)
+
+	c.Assert(im.Search(id1).GetIdentity(), qt.Equals, id1)
+	c.Assert(im.Search(testIdentity{name: "notfound"}), qt.Equals, nil)
+}
+
+type testIdentity struct {
+	name string
+}
+
+func (id testIdentity) GetIdentity() Identity {
+	return id
+}
+
+func (id testIdentity) Name() string {
+	return id.name
+}