use appropriate `time_type` and format of time values in preview/plot/export data

[melange396]

The "Preview data", "Plot data", and "Export data" functionality on the individual signal pages work well for data with a time_type of "day", but not for signals that have a "week" type...

• for "preview data" and "export data" modes, startDate and endDate should be converted to a corresponding EpiWeek value (with format YYYYWW, like "200051" for the 51st week of the year 2000)

  • delphi_utils uses the epiweeks package for this

• for "plot data" mode, this should use "day" or "week" as appropriate for the signal:
  

  signal_documentation/src/templates/signals/data_visualization_export.html

  Line 250 in 9f11821

  var urlParamsEncoded = btoa(`{"datasets":[{"color":"#415742","title":"value","params":{"_endpoint":"covidcast","data_source":"${dataSource}","signal":"${dataSignal}","time_type":"day","geo_type":"${geographicType}","geo_value":"${geographicValue}"}}]}`);

  as the "preview data" mode already does:

  signal_documentation/src/templates/signals/data_visualization_export.html

  Line 269 in 9f11821

  var timeType = document.getElementById('time_type').value;

  (note: time_type of "week" is not yet fully supported in epivis)

[dmytrotsko]

Solved this issue in the latest PR.