Closes 10759: Enable markdown support for custom field descriptions

[decoupca]

Fixes: #10759

Enables markdown support for:

• Custom field descriptions
• Custom validator error messages

[jeremystretch]

I agree with supporting Markdown rendering for custom field descriptions, but I see no need to implement it for form errors: These values are not user-configurable. (It would need to be a separate FR anyway, as the change would impact far more than just custom fields.)

[decoupca]

@jeremystretch

I agree with supporting Markdown rendering for custom field descriptions, but I see no need to implement it for form errors: These values are not user-configurable. (It would need to be a separate FR anyway, as the change would impact far more than just custom fields.)

Custom validator error messages are user-configurable, which in my case would benefit from markdown support (linking to internal KBs, etc.) Code in this PR achieves that, which didn't look like it had a large impact. Maybe I missed something?

[jeremystretch]

There are a lot more places we'd need to apply the Markdown filter for form errors, and some degree of cleanup is needed if we want to head down that path. Let's prune down #10759 to cover just the custom field descriptions so that we can implement that change, then tackle form errors in general under a separate FR.

[decoupca]

@jeremystretch copy that, reverted changes.

[jeremystretch]

Thanks @decoupca!