Mobile: credit card number input field should trigger number keypad

[KFie]

Environment: mobile

Reproduce:

1. Add random item to cart
2. Start checkout until you arrive at payment step
3. Tap into credit card number field

Issue: the credit card field doesn't trigger the number keypad. This is custom in real-life stores and should be done here as well. Applies to payment section in account as well.

May be worth considering for postal/zip code field as well if countries remain DE and US.

[rschwietzke]

Might be fixes due to the change to pure Bootstrap, let's test it

[freddyka]

Issue not fixed for poster v2.1.

-->reason is that mobile menu button can't be used (see#199)