Add input templating function and specific template for custom date range

[kevinfodness]

• Adds a generic templating function called input that can be used on any aggregation to output a recommended set of form controls for the aggregation, which can be overridden by individual aggregations to customize the form fields.
• Sets the default for all aggregations to checkboxes.
• Defaults the relative date aggregation to a select.
• Adds a custom set of inputs for the custom date range aggregation's input function which handles date selection using a built-in date input and a hidden field to track the ISO-8601 date separately from the date input.

[vancoder]

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