support SearchAPI date fields

Problem/Motivation

The calendar plugins don't work with SearchAPI date fields.

AFAICT this wouldn't need much work:

- isDateField() needs to detect SearchAPI date fields
- getDateFieldValues() needs to extract a date from such a field

I've got this working with my search index with some rough hacking as a proof of concept.

What I'm not sure about is how to detect date fields -- my field is not using the SearchApiDate field plugin but the SearchApiStandard and I don't know why.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes