Add topic support (#3711)

* add topic models and unit tests

* fix comments

* fix comment

* add the UI to show or add topics for a repo

* show topics on repositories list

* fix test

* don't show manage topics link when no permission

* use green basic as topic label

* fix topic label color

* remove trace content

* remove debug function

routers/repo/topic.go

@@ -0,0 +1,38 @@
+// Copyright 2018 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 repo
+
+import (
+	"strings"
+
+	"code.gitea.io/gitea/models"
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/log"
+)
+
+// TopicPost response for creating repository
+func TopicPost(ctx *context.Context) {
+	if ctx.User == nil {
+		ctx.JSON(403, map[string]interface{}{
+			"message": "Only owners could change the topics.",
+		})
+		return
+	}
+
+	topics := strings.Split(ctx.Query("topics"), ",")
+
+	err := models.SaveTopics(ctx.Repo.Repository.ID, topics...)
+	if err != nil {
+		log.Error(2, "SaveTopics failed: %v", err)
+		ctx.JSON(500, map[string]interface{}{
+			"message": "Save topics failed.",
+		})
+		return
+	}
+
+	ctx.JSON(200, map[string]interface{}{
+		"status": "ok",
+	})
+}