Clean stale PRs and bring this repo to a finalized stable state

[dshemetov]

This is a meta issue for clean-up work in this repo. The goal is to clean out old work that never got merged and then only bugfix (no new features).

Cleaning out old PRs:

• Flip lag parity for python covidcast #524
• Add PR to fips<>name and territories to fips<>abbr/ fips<>name to Python package #497
• housekeeping: rename api param source -> endpoint #379
• Add title argument to plotting function #363
• Add axes argument and return axes in plots #361
• The rest of the PRs

Other issues:

• Merge individual package branches to main #625
• fix package installation bug(evalcast): remove title from labs in plots #641

Other tasks:

• Update CI to test R 3.5 and bring evalcast requirements back down to >3.5 if tests pass