refactor(): Change naming of PayPal Icons

The official payment icon should always be the default. Former
default icons should get a modifier (written in BEM). By this
way we can add another modifier, a PayPal compact icon for example
with just the two first letters.

Old:

```css
.pw-paypal-new:before {}
.pw-paypal:before {}
```

New:

```css
.pw-paypal:before {}
.pw-paypal--classic:before {}
.pw-paypal--compact:before {}
```