[Merge] lp:~jks/mailman/hcaptcha into lp:mailman/2.1

Jouni K. Seppänen has proposed merging lp:~jks/mailman/hcaptcha into lp:mailman/2.1.
Requested reviews: Mailman Coders (mailman-coders)
For more details, see: https://code.launchpad.net/~jks/mailman/hcaptcha/+merge/389691
I noticed that a Mailman 2.1 site I run was being used for sending subscribe confirmation messages to people. I didn't want to implement reCaptcha because of privacy concerns, and hCaptcha seems to be almost a drop-in replacement that at least promises to respect user privacy. This change allows using hCaptcha instead of reCaptcha for guarding the subscription form.
When testing, the dummy keys from https://docs.hcaptcha.com/#localdev may be useful:
Dummy Site Key:10000000-ffff-ffff-ffff-000000000001 Dummy Secret Key:0x0000000000000000000000000000000000000000
participants (1)
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Jouni K. Seppänen