[UI] Convert milestone to HTMX

- Currently if you want to update the milestone of an issue or pull
request, your whole page will be reloaded to reflect the newly set
milestone. This is quite unecessary, as only the milestone text is
updated and a new timeline event is added.
- This patch converts the milestone section in the issue/pull request
sidebar to use HTMX, so it becomes a progressive element and avoids
reloading the whole page to update the milestone.
- The update of the milestone section itself is quite straightforward
and nothing special is happening. To support adding new timeline events,
a new element `#insert-timeline` is conviently placed after the last
timeline event, which can be used with
[`hx-swap-oob`](https://htmx.org/attributes/hx-swap-oob/) to position
new timeline events before that element.
- Adds E2E test.

routers/web/repo/issue.go

@@ -1370,6 +1370,22 @@ func getBranchData(ctx *context.Context, issue *issues_model.Issue) {
 	}
 }
 
+func prepareHiddenCommentType(ctx *context.Context) {
+	var hiddenCommentTypes *big.Int
+	if ctx.IsSigned {
+		val, err := user_model.GetUserSetting(ctx, ctx.Doer.ID, user_model.SettingsKeyHiddenCommentTypes)
+		if err != nil {
+			ctx.ServerError("GetUserSetting", err)
+			return
+		}
+		hiddenCommentTypes, _ = new(big.Int).SetString(val, 10) // we can safely ignore the failed conversion here
+	}
+
+	ctx.Data["ShouldShowCommentType"] = func(commentType issues_model.CommentType) bool {
+		return hiddenCommentTypes == nil || hiddenCommentTypes.Bit(int(commentType)) == 0
+	}
+}
+
 // ViewIssue render issue view page
 func ViewIssue(ctx *context.Context) {
 	if ctx.Params(":type") == "issues" {
@@ -2019,21 +2035,13 @@ func ViewIssue(ctx *context.Context) {
 	ctx.Data["NewPinAllowed"] = pinAllowed
 	ctx.Data["PinEnabled"] = setting.Repository.Issue.MaxPinned != 0
 
-	var hiddenCommentTypes *big.Int
-	if ctx.IsSigned {
-		val, err := user_model.GetUserSetting(ctx, ctx.Doer.ID, user_model.SettingsKeyHiddenCommentTypes)
-		if err != nil {
-			ctx.ServerError("GetUserSetting", err)
-			return
-		}
-		hiddenCommentTypes, _ = new(big.Int).SetString(val, 10) // we can safely ignore the failed conversion here
-	}
-	ctx.Data["ShouldShowCommentType"] = func(commentType issues_model.CommentType) bool {
-		return hiddenCommentTypes == nil || hiddenCommentTypes.Bit(int(commentType)) == 0
+	prepareHiddenCommentType(ctx)
+	if ctx.Written() {
+		return
 	}
+
 	// For sidebar
 	PrepareBranchList(ctx)
-
 	if ctx.Written() {
 		return
 	}
@@ -2342,7 +2350,49 @@ func UpdateIssueMilestone(ctx *context.Context) {
 		}
 	}
 
-	ctx.JSONOK()
+	if ctx.FormBool("htmx") {
+		renderMilestones(ctx)
+		if ctx.Written() {
+			return
+		}
+		prepareHiddenCommentType(ctx)
+		if ctx.Written() {
+			return
+		}
+
+		issue := issues[0]
+		var err error
+		if issue.MilestoneID > 0 {
+			issue.Milestone, err = issues_model.GetMilestoneByRepoID(ctx, ctx.Repo.Repository.ID, issue.MilestoneID)
+			if err != nil {
+				ctx.ServerError("GetMilestoneByRepoID", err)
+				return
+			}
+		} else {
+			issue.Milestone = nil
+		}
+
+		comment := &issues_model.Comment{}
+		has, err := db.GetEngine(ctx).Where("issue_id = ? AND type = ?", issue.ID, issues_model.CommentTypeMilestone).OrderBy("id DESC").Limit(1).Get(comment)
+		if !has || err != nil {
+			ctx.ServerError("GetLatestMilestoneComment", err)
+		}
+		if err := comment.LoadMilestone(ctx); err != nil {
+			ctx.ServerError("LoadMilestone", err)
+			return
+		}
+		if err := comment.LoadPoster(ctx); err != nil {
+			ctx.ServerError("LoadPoster", err)
+			return
+		}
+		issue.Comments = issues_model.CommentList{comment}
+
+		ctx.Data["Issue"] = issue
+		ctx.Data["HasIssuesOrPullsWritePermission"] = ctx.Repo.CanWriteIssuesOrPulls(issue.IsPull)
+		ctx.HTML(http.StatusOK, "htmx/milestone_sidebar")
+	} else {
+		ctx.JSONOK()
+	}
 }
 
 // UpdateIssueAssignee change issue's or pull's assignee