re-enable markdown in custom columns

[chcon]

Fixes: #9979

Introduced a new column class CustomFieldMarkdownColumn that combines the default markdown rendering behavior of the MarkdownColumn with the custom column specific constructor parameters of the CustomFieldColumn. That way markdown in custom columns should be both XSS safe and functional.

[jeremystretch]

Thanks for taking this on @chcon. We should be able to avoid introducing a second column class specifically for Markdown support if we can just adjust the rendering logic for longtext-type fields in the existing class.

[chcon]

@jeremystretch agreed, please check it again.