Support date in Timeline selection in Grapher

[lucasrodes]

Core problem

In ETL, we sometimes work with time series with dates (not years). However, Grapher uses 'number of days since zeroDay' under the hood, and is not able to select dates in the Timeline selection.

Proposed solution

It'd be nice if we could render a date selector there. Or support date-like strings.

Context

For Grapher to support date format, we have been using a hack:

• Before adding indicator data with dates to Grapher, convert to 'a number of days passed since [day of reference].'
• This conversion was done manually by computing DATE_REFERENCE - date in days. Then, we filled the metadata fields yearIsDay=True and zeroDay=DATE_REFERENCE.
• This was recently improved (see https://github.com/owid/etl/blob/1e261241e77b1717e975c9c2273a60e49867707a/etl/grapher_helpers.py#L750) and is now done automatically in ETL. As DATE_REFERENCE, we are using the minimum date appearing in the time series. However, this field is exposed in ETL and hence can be changed by the user. So, we it could be any date.

[lucasrodes]

Saloni was confused with the Timeline selection in https://github.com/owid/owid-issues/issues/1653