Permit inequalities for `issue`, `time_value`

[krivard]

With #1070 we now support time_value=* and issue=* in covidcast requests, which prevents users from having to specify ridiculous ranges like 11111111-99999999 in their queries. But if a user wants an open-ended portion of the complete calendar or history, they still have to put in silly values.

Could we support inequalities like the below?

• all issues on or before some date
• all issues on or after some date
• all time_values on or before some date
• all time_values on or after some date

I'm not sure what a good syntax would be. Some ideas are:

• <YYYYmmdd <=YYYYmmdd >YYYYmmdd >=YYYYmmdd
• *-YYYYmmdd YYYYmmdd-*

We'd then have the following mapping to SQL queries:

meaning	parameter syntax	SQL
one date	YYYYmmdd or YYYY-mm-dd	issue = YYYYmmdd or time_value = YYYYmmdd
range	YYYYmmdd-YYYYmmdd or YYYY-mm-dd--YYYY-mm-dd	issue BETWEEN YYYYmmdd AND YYYYmmdd or time_value BETWEEN YYYYmmdd AND YYYYmmdd
any	*	issue or time_value not included in WHERE clause
open-ended	TBD	issue < YYYYmmdd or issue <= YYYYmmdd or issue > YYYYmmdd or issue >= YYYYmmdd or similar for time_value