🔧 Allow conference chairs; remove jats refs in abstracts

[fwkoch]

Just a couple small fixes:

• Conference proceedings have "contributors" - we pull these from myst.yml#project.editors. Previously they were always marked as "editors" - now there's an option in the CLI to mark these contributors as "chairs"
• Cross-references are removed from abstracts. These are not necessary for crossref deposits and can easily break the schema validation.