Allow multiple fields for dates

[jeremyboggs]

There are plenty of places in an item's metadata that can store dates. Additionally, the Dublin Core Extended plugin adds a variety of more granular date fields, all of which could be used to display an item on the timeline. Seems useful to add some way for users to choose multiple fields for Neatline Time to read for dates.

At its simplest, this could involve providing a way to do multiple selects for the Date field in the plugin options panel.

[jeremyboggs]

If this is implemented, it would seem useful to add some information to the item bubble about which date field is actually being used for that particular marker.

[ad2114]

+1 to this request.

[erochest]

Rather than multiple date fields, which would have weird, tangled semantics, allow selecting alternative date fields.

We'd need:

• schema change to have each exhibit define its own date field;
• migration;
• change the exhibit config form to prompt for the date field; and
• change to the query to look in the right field.