Migrate all indicators to a common export function

[sgsmob]

We have a lot of replication of export code across indicators. It doesn't appear that the functionality is actually that different across indicators, so we should build out delphi_utils.create_export_csv() and migrate all the indicators to use it.

Indicators that still need to be migrated:

• google_health
• nchs_mortality
• quidel
• quidel_covidtest
• safegraph (Refactor safegraph to use export util #574)
• safegraph_patterns

[chinandrew]

safegraph_patterns looks like it used the original create_export_csv implementation, so checking it off

[chinandrew]

nchs_mortality uses epiweeks instead of days, so punting for now

[dshemetov]

Been diving into the changehc code recently. Seems like it could move to using this utility, at first glance. Would mostly need to pack the existing row data into a dataframe at the end of update_sensor and replace write_to_csv with a call to the export util. Thoughts @rumackaaron?