Improve Captcha integration

Problem/Motivation

The captcha element is being saved to the database as a part of YAML form submission. The captcha element should be ignored since it is only intended to verify a user and the actual posted value is irrelevant.

Proposed resolution

• Create a YamlFormElementHandler for captcha which extend ContainerBase.
• Filter out posted captcha data from a YAML form submission.
• Look into hiding the captcha element for authenticated users.
• Write tests for new YamlFormElementHandler hooks.

API changes

• Most likely need to add hooks to the YamLFormElementHandler plugin.

Development Notes

• Branch Name: 2742229-improve-captcha-integration
• Patch Name: yamlform-improve-captcha-integration-2742229-XX.patch
• Commit message: Issue #2742229: Improve Captcha integration.