Provide a generic "Datasource" StandardCustomField

Summary:
Ref T9253. See discussion in D13843.

I want to let Drydock blueprints for Almanac services choose those services from a typeahead, but only list appropriate services in the typeahead. To do this:

• Provide a StandardCustomField for an arbitrary datasource.
• Adjust the AlmanacServiceDatasource to allow filtering by service class.

This implementation is substantially the same as the one in D13843, with some adjustments:

• I lifted most of the code in the Users standard custom field into a new Tokenizer standard custom field.
• The Users and Datasource custom fields now extend the Tokenizer custom field and can share most of the code it uses.
• I exposed this field fully as a configurable field. I don't think anyone will ever use it, but this generality costs us nearly nothing and improves consistency.
• The code in D13843 didn't actually pass the parameters over the wire, since the object that responds to the request is not the same object that renders the field. Use the "parameters" mechanism in datasources to get things passed over the wire.

Test Plan:

• Created a custom "users" field in Maniphest and made sure it still wokred.
• Created a custom "almanc services" field in Maniphest and selected some services for a task.
• With additional changes from D13843, selected an appropriate Almanac service in a new Drydock blueprint.

Reviewers: hach-que, chad

Reviewed By: hach-que, chad

Maniphest Tasks: T9253

Differential Revision: https://secure.phabricator.com/D14111