Add settings for document tracking

Problem/Motivation

The `documentCls`, `documentIgnoreCls`, and `documentExt` fields in the settings form were not available in the module but I needed them for my project.

Changes

- enabled `documentCls` and `documentIgnoreCls` fields in settings form
- added `documentExt` field to settings form
- added field validation for `documentExt` field in `MonsidoSettingsForm` class
- added configs for `documentCls`, `documentIgnoreCls`, and `documentExt`
- updated the attached `drupalSettings`
- updated the `monsido.js` object