Fix bootstrap3 datepicker filter

When using bootstrap3, the datepicker filter doesn't work because it's using the bootstrap3 datepicker default format rather than the format that flask-admin is expecting.

1. form_js()'s form.js needs to load after filters.js, because filters.js makes changes to the field before it's initialized

2. The datepicker widget is set to parse YYYY-MM-DD by default: https://github.com/mrjoes/flask-admin/blob/d0b60696d5983d2f06c90b351fca7c0cd9d1eccc/flask_admin/form/widgets.py#L51 I'm not 100% sure this is the code applied applied when the date filter is parsed, but it does seem to parse only YYYY-MM-DD. So, I hardcoded data-date-format to YYYY-MM-DD in filters.js and it works. We probably need to make this more flexible somehow.