Save and view issue/comment content history (#16909)

* issue content history

* Use timeutil.TimeStampNow() for content history time instead of issue/comment.UpdatedUnix (which are not updated in time)

* i18n for frontend

* refactor

* clean up

* fix refactor

* re-format

* temp refactor

* follow db refactor

* rename IssueContentHistory to ContentHistory, remove empty model tags

* fix html

* use avatar refactor to generate avatar url

* add unit test, keep at most 20 history revisions.

* re-format

* syntax nit

* Add issue content history table

* Update models/migrations/v197.go

Co-authored-by: 6543 <6543@obermui.de>

* fix merge

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>

models/issues/main_test.go

@@ -0,0 +1,16 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package issues
+
+import (
+	"path/filepath"
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+)
+
+func TestMain(m *testing.M) {
+	db.MainTest(m, filepath.Join("..", ".."), "")
+}