Add "Purpose of transfer" to the donation page bank transfer section

Suggested by sycamoreone on the testers mailinglist:

some feedback from actual donors (before the campaign ends): For a bank
transfer (like to Zwiebelfreunde e.V.) people are usually given a
“Purpose of transfer” or “Verwendungszweck”. It is not obvious that the
purpose of the transfer is encoded into the IBAN so people are confused
and probably reluctant to just transfer the money.

For larger donations it would also be interesting to get information
about the possibility of a tax refund (because Zwiebelfreunde is
registered as a not-for-profit in Germany). I think the old donation
pages mentioned that one needs to contact Zwiebelfreunde for this.

I agree, that if we keep the new donation page, which we actually could, we should add this information back there.

Feature Branch: 451f:tails/feature/12155+transfer_purpose

Parent Task: #12035 (closed)

Related issues

• Related to #12036 (closed)

Original created by @u on 12155 (Redmine)