Resolve "Cleanup invisible_captcha feature flag"

What does this MR do?

This turns the invisible_captcha feature flag into a invisible_captcha_enabled application setting.

It also updates the gem to the latest version.

After deploying:

• run /chatops run feature delete invisible_captcha in production channel
• ask an admin to enable Settings > Reporting > Spam and Anti-bot Protection > Enable Invisible Captcha during sign up

Note that after this has been deployed and before the setting has been turned on, invisible captcha will be disabled for a brief time. This is of no real concern.

Output of rails db:migrate:redo:

== 20201229105948 AddInvisibleCaptchaEnabledToSettings: reverting =============
-- remove_column(:application_settings, :invisible_captcha_enabled, :boolean, {:null=>false, :default=>false})
   -> 0.0139s
== 20201229105948 AddInvisibleCaptchaEnabledToSettings: reverted (0.0187s) ====

== 20201229105948 AddInvisibleCaptchaEnabledToSettings: migrating =============
-- add_column(:application_settings, :invisible_captcha_enabled, :boolean, {:null=>false, :default=>false})
   -> 0.0062s
== 20201229105948 AddInvisibleCaptchaEnabledToSettings: migrated (0.0063s) ====

Screenshots (strongly suggested)

Related to #284113 (closed)