[MM-62092] Allow system admins to pull posts in from DMs they're not in (#29557)

server/channels/api4/post.go

@@ -214,7 +214,7 @@ func getPostsForChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.Err = err
 		return
 	}
-	if !c.App.SessionHasPermissionToReadChannel(c.AppContext, *c.AppContext.Session(), channel) {
+	if !c.IsSystemAdmin() && !c.App.SessionHasPermissionToReadChannel(c.AppContext, *c.AppContext.Session(), channel) {
 		c.SetPermissionError(model.PermissionReadChannelContent)
 		return
 	}