Unify setup of expiration date selection

Extend the helper for setting up a date combo box for entering an
expiration date for usage in the certification dialog:

• Allow specifying a custom date range.
• Set the upper limit to the technical limit if no upper limit is specified.
• Enable the warnings on invalid date.
• Add a few predefined values for the validity period.

• GnuPG-bug-id: T6621