Show outdated comments in files changed tab (#24936) (#25428)

Backport #24936

If enabled show a clickable label in the comment. A click on the label
opens the Conversation tab with the comment focussed - there you're able
to view the old diff (or original diff the comment was created on).

**Screenshots**


![image](63ab9571-a9ee-4900-9f02-94ab0095f9e7)


![image](78f7c225-8d76-46f5-acfd-9b8aab988a6c)

When resolved and outdated:


![image](6ece9ebd-c792-4aa5-9c35-628694e9d093)

Option to enable/disable this (stored in user settings - default is
disabled):


![image](ed99dfe4-76dc-4c12-bd96-e7e62da50ab5)


![image](e837a052-e92e-4a28-906d-9db5bacf93a6)

fixes #24913

Co-authored-by: silverwind <me@silverwind.io>

routers/web/repo/middlewares.go

@@ -5,6 +5,7 @@ package repo
 
 import (
 	"fmt"
+	"strconv"
 
 	system_model "code.gitea.io/gitea/models/system"
 	user_model "code.gitea.io/gitea/models/user"
@@ -88,3 +89,27 @@ func SetWhitespaceBehavior(ctx *context.Context) {
 		ctx.Data["WhitespaceBehavior"] = whitespaceBehavior
 	}
 }
+
+// SetShowOutdatedComments set the show outdated comments option as context variable
+func SetShowOutdatedComments(ctx *context.Context) {
+	showOutdatedCommentsValue := ctx.FormString("show-outdated")
+	// var showOutdatedCommentsValue string
+
+	if showOutdatedCommentsValue != "true" && showOutdatedCommentsValue != "false" {
+		// invalid or no value for this form string -> use default or stored user setting
+		if ctx.IsSigned {
+			showOutdatedCommentsValue, _ = user_model.GetUserSetting(ctx.Doer.ID, user_model.SettingsKeyShowOutdatedComments, "false")
+		} else {
+			// not logged in user -> use the default value
+			showOutdatedCommentsValue = "false"
+		}
+	} else {
+		// valid value -> update user setting if user is logged in
+		if ctx.IsSigned {
+			_ = user_model.SetUserSetting(ctx.Doer.ID, user_model.SettingsKeyShowOutdatedComments, showOutdatedCommentsValue)
+		}
+	}
+
+	showOutdatedComments, _ := strconv.ParseBool(showOutdatedCommentsValue)
+	ctx.Data["ShowOutdatedComments"] = showOutdatedComments
+}