Add IBAN support

[eivindkopperud]

If we get enough requests for IBAN support, we should add some validation for IBAN too. That depends on us having a lot of international users, however, and there are not too many of them in group boards.

We can use this package for IBAN validation:

https://www.npmjs.com/package/iban used this package when working with bank, it works great :)

Originally posted by @Mortefal in #24 (comment)

[Mortefal]

This package also verifies BBAN, so that the accountnumber actually is valid

[eivindkopperud]

Aha!! And BBAN is the standard we use for Norwegian bank accounts?

[Mortefal]

Yeah :)