Add MAX_ROWS option for CSV rendering (#30268)

This solution implements a new config variable MAX_ROWS, which
corresponds to the “Maximum allowed rows to render CSV files. (0 for no
limit)” and rewrites the Render function for CSV files in markup module.
Now the render function only reads the file once, having MAX_FILE_SIZE+1
as a reader limit and MAX_ROWS as a row limit. When the file is larger
than MAX_FILE_SIZE or has more rows than MAX_ROWS, it only renders until
the limit, and displays a user-friendly warning informing that the
rendered data is not complete, in the user's language.

---

Previously, when a CSV file was larger than the limit, the render
function lost its function to render the code. There were also multiple
reads to the file, in order to determine its size and render or
pre-render.

The warning: ![image](https://s3.amazonaws.com/i.snag.gy/vcKh90.jpg)

modules/setting/ui.go

@@ -52,6 +52,7 @@ var UI = struct {
 
 	CSV struct {
 		MaxFileSize int64
+		MaxRows     int
 	} `ini:"ui.csv"`
 
 	Admin struct {
@@ -107,8 +108,10 @@ var UI = struct {
 	},
 	CSV: struct {
 		MaxFileSize int64
+		MaxRows     int
 	}{
 		MaxFileSize: 524288,
+		MaxRows:     2500,
 	},
 	Admin: struct {
 		UserPagingNum   int