Table should use Bootstrap icons

[Pander]

The icons in https://github.com/greyli/bootstrap-flask/tree/master/flask_bootstrap/static/img should be removed and the table macro should use the following Bootstrap icons:

• https://icons.getbootstrap.com/icons/eye-fill/
• https://icons.getbootstrap.com/icons/pencil-fill/
• https://icons.getbootstrap.com/icons/trash-fill/

As you can see the icons are already at a newer and improved version.