Polish the Bootstrap theme

[limonte]

This graph is showing that Bootstrap 4 theme is the absolute winner :)

@limonte spend some time testing it extensively and verifying all details are aligned with Bootstrap 4 modal: https://getbootstrap.com/docs/4.0/components/modal/

[gverni]

It's funny how the theme that took us longer to implement (material-theme) is actually trailing behind the others. I feel we need to start working on documenting theme on sweetalert.github.io and see if we get more traction. I'll start working on sweetalert2/sweetalert2.github.io#79

[limonte]

I'll start working on sweetalert2/sweetalert2.github.io#79

Thank you, that's the important one!