Captcha for supporting.openstreetmap.org forms

[Firefishy]

There are a number of forms on supporting.openstreetmap.org which email the form contents to osmfoundation.org.

Many submissions from these forms are spam (bots?), the form spam contents are emailed via OpenStreetMap.org's outgoing mail server. These emails influence our domain and IP spam reputation.

We should add a captcha and/or other anti spam controls to these forms to reduce the amount of spam sent via these forms.

Example forms:
https://supporting.openstreetmap.org/active-contributor-membership/application-form-for-active-contributor-membership-other/
https://supporting.openstreetmap.org/alternative-payment-options/