Mention completion for issue editor. (#3136)

* new issue mention

* Mention completion on new issue and view issue page.

* Code format.

* Require tribute in pull request page.

routers/repo/pull.go

@@ -701,6 +701,7 @@ func CompareAndPullRequest(ctx *context.Context) {
 	ctx.Data["PageIsComparePull"] = true
 	ctx.Data["IsDiffCompare"] = true
 	ctx.Data["RequireHighlightJS"] = true
+	ctx.Data["RequireTribute"] = true
 	setTemplateIfExists(ctx, pullRequestTemplateKey, pullRequestTemplateCandidates)
 	renderAttachmentSettings(ctx)