Use a CAPTCHA when registering new accounts

**Is your feature request related to a problem? Please describe.**
The community faucet was being abused, see the account activity at https://wallet.bitshares.org/#/account/onboarding.bitshares.foundation.

**Describe the solution you'd like**
One way to fight that kind of abuse is CAPTCHA. Note that the UI doesn't support CAPTCHA so far, so it's not a job that could be done by the faucet only, but collaboration is needed.

**Describe alternatives you've considered**
* 2nd-step verification e.g. email / mobile phone / social media
* pay (via a payment processor) for registration

**Additional context**
Add any other context or screenshots about the feature request here.

Start by reviewing the account-registration UI and how the community faucet invokes registration, then identify the existing UI and registration boundaries involved. Compare CAPTCHA with the listed email, mobile, social-media, and payment alternatives; done should be defined through a decided anti-abuse approach that the UI and faucet can support together.

Written by the indexing model from the issue text.