update covidcast 'firstDate'

[melange396]

update firstDate used with the covidcast endpoint, to account for new, recently backfilled google-symptoms data.

closes #92

date references:

• patch google symptom early 2017-08-15 - 2020-02-19 covidcast-indicators#2107 (comment)
• https://github.com/cmu-delphi/delphi-epidata/blob/65809bab5735cf3f4836ed5ce2cbeeb845c7745e/docs/api/covidcast-signals/google-symptoms.md?plain=1#L57-L71

[melange396]

compare the preview site with the current live site, both with the same post-hash encoding to show google-symptoms:s01_raw_search, and notice that the current does not show pre-2020 data:

• https://deploy-preview-108--cmu-delphi-epivis.netlify.app/#eyJjaGFydCI6eyJ2aWV3cG9ydCI6WzczNjU1NC41LDAuMDA4MzMzMzMzMzMzMzMzMzA0LDczOTMxNy41LDUuMTQxNjY2NjY2NjY2NjY3NV0sInNob3dQb2ludHMiOmZhbHNlfSwiZGF0YXNldHMiOlt7ImNvbG9yIjoiI2EzMzBjZCIsInRpdGxlIjoidmFsdWUiLCJwYXJhbXMiOnsiX2VuZHBvaW50IjoiY292aWRjYXN0IiwiZGF0YV9zb3VyY2UiOiJnb29nbGUtc3ltcHRvbXMiLCJzaWduYWwiOiJzMDFfcmF3X3NlYXJjaCIsInRpbWVfdHlwZSI6ImRheSIsImdlb190eXBlIjoic3RhdGUiLCJnZW9fdmFsdWUiOiJwYSJ9fV19
• https://delphi.cmu.edu/epivis/#eyJjaGFydCI6eyJ2aWV3cG9ydCI6WzczNjU1NC41LDAuMDA4MzMzMzMzMzMzMzMzMzA0LDczOTMxNy41LDUuMTQxNjY2NjY2NjY2NjY3NV0sInNob3dQb2ludHMiOmZhbHNlfSwiZGF0YXNldHMiOlt7ImNvbG9yIjoiI2EzMzBjZCIsInRpdGxlIjoidmFsdWUiLCJwYXJhbXMiOnsiX2VuZHBvaW50IjoiY292aWRjYXN0IiwiZGF0YV9zb3VyY2UiOiJnb29nbGUtc3ltcHRvbXMiLCJzaWduYWwiOiJzMDFfcmF3X3NlYXJjaCIsInRpbWVfdHlwZSI6ImRheSIsImdlb190eXBlIjoic3RhdGUiLCJnZW9fdmFsdWUiOiJwYSJ9fV19

[nmdefries]

This looks fine, but I wonder why we use the firstDate param at all. I assume we always want to show the full history of an indicator.

[melange396]

@nmdefries : you raise a good point that i, myself, have wondered. The "first date" concept precedes the big svelete rewrite in May 2021, and even the 2nd commit to this repo which imported the old epivis code in November 2016:

www-epivis/site/js/epidata.js

Lines 5 to 25 in 3c07bad

	// first available epiweek for each data source
	var first_epiweek = {
	'fluview': 199740,
	'ilinet': 199740,
	'stateili': 201001,
	'gft': 200340,
	'ght': 200401,
	'twitter': 201148,
	'wiki': 200750,
	'nidss_flu': 200814,
	'nidss_dengue': 200301,
	'signals': 200340,
	'cdc': 201301,
	'sensors': 201030,
	'nowcast': 200901,
	};
	// first available date for each data source
	var first_date = {
	'twitter': 20111201,
	'wiki': 20071209,
	};

It mightve been a requirement from how older versions of endpoints and/or indexes worked, or just a premature optimization.