Add optional hCaptcha support

Fixes #1649

This requires setting `HCAPTCHA_SECRET_KEY` and `HCAPTCHA_SITE_KEY`, then
enabling the admin setting at
`/admin/settings/edit#form_admin_settings_captcha_enabled`

Subsequently, a hCaptcha widget will be displayed on `/about` and
`/auth/sign_up` unless:
- the user is already signed-up already
- the user has used an invite link
- the user has already solved the captcha (and registration failed for another
  reason)

The Content-Security-Policy headers are altered automatically to allow the
third-party hCaptcha scripts on `/about` and `/auth/sign_up` following the same
rules as above.

1 files changed, 2 insertions, 0 deletions

diff --git a/Gemfile b/Gemfile
index eae5f11b7..282ab65e4 100644
--- a/Gemfile
+++ b/Gemfile
@@ -156,3 +156,5 @@ gem 'concurrent-ruby', require: false
 gem 'connection_pool', require: false
 
 gem 'xorcist', '~> 1.1'
+
+gem "hcaptcha", "~> 7.1"