Refine publication date logic

[Aetf]

From SymbioticLab/SymbioticLab.github.io#219 (comment):

The current logic is

• Use date from publication's conference if present
• Use year and month from publication

This could be enhanced to be

• Use date field (as spec'd in biblatex manual, section 2.3.8) from publication
• Use date from publication's conference if present
• Use year and month from publication (this is put at last because they are deprecated fields)

This way we still get a default date for sorting for publications associated with a conference, but we can additionally fine tune the date for individual publication entries.