Prompt Team Admin before joining private channel (#29955)

* Prompt Team Admin before joining private channel

* fix and implement prompt when team admin joins via channel link

* update premission check and unit tests

* remove comment

* clean up some code

* update check to fix E2E tests

server/channels/api4/channel.go

@@ -1406,7 +1406,9 @@ func getChannelByName(c *Context, w http.ResponseWriter, r *http.Request) {
 			return
 		}
 	} else {
-		if !c.App.SessionHasPermissionToChannel(c.AppContext, *c.AppContext.Session(), channel.Id, model.PermissionReadChannel) {
+		// allows team admins to access private channel
+		if !c.App.SessionHasPermissionToTeam(*c.AppContext.Session(), channel.TeamId, model.PermissionManageTeam) &&
+			!c.App.SessionHasPermissionToChannel(c.AppContext, *c.AppContext.Session(), channel.Id, model.PermissionReadChannel) {
 			c.Err = model.NewAppError("getChannelByName", "app.channel.get_by_name.missing.app_error", nil, "teamId="+channel.TeamId+", "+"name="+channel.Name+"", http.StatusNotFound)
 			return
 		}
@@ -1436,17 +1438,19 @@ func getChannelByNameForTeamName(c *Context, w http.ResponseWriter, r *http.Requ
 		return
 	}
 
-	teamOk := c.App.SessionHasPermissionToTeam(*c.AppContext.Session(), channel.TeamId, model.PermissionReadPublicChannel)
 	channelOk := c.App.SessionHasPermissionToChannel(c.AppContext, *c.AppContext.Session(), channel.Id, model.PermissionReadChannel)
-
 	if channel.Type == model.ChannelTypeOpen {
+		teamOk := c.App.SessionHasPermissionToTeam(*c.AppContext.Session(), channel.TeamId, model.PermissionReadPublicChannel)
 		if !teamOk && !channelOk {
 			c.SetPermissionError(model.PermissionReadPublicChannel)
 			return
 		}
 	} else if !channelOk {
-		c.Err = model.NewAppError("getChannelByNameForTeamName", "app.channel.get_by_name.missing.app_error", nil, "teamId="+channel.TeamId+", "+"name="+channel.Name+"", http.StatusNotFound)
-		return
+		// allows team admins to access private channel
+		if !c.App.SessionHasPermissionToTeam(*c.AppContext.Session(), channel.TeamId, model.PermissionManageTeam) {
+			c.Err = model.NewAppError("getChannelByNameForTeamName", "app.channel.get_by_name.missing.app_error", nil, "teamId="+channel.TeamId+", "+"name="+channel.Name+"", http.StatusNotFound)
+			return
+		}
 	}
 
 	appErr = c.App.FillInChannelProps(c.AppContext, channel)