Package app cleanup

[seankross]

Description

• Adds basic GitHub Actions based CI/CD infrastructure and related badging.
• Adds example_callback() to help folks who are deploying this package understand what data they need to provide to the app.
• Adds oncarto_file() to help access files that are packaged with the app.
• The county-incidence tab now uses the best-practice pattern of putting the conditional logic within the renderLeaflet function call.
• Adds WILDS project status badging.
• Modifies tests to use example_callback().

Example

oncarto::oncarto_file() |> basename() |> paste0(collapse = "\n") |> cat()
#> fh-contact-info.md
#> fh-logo.png
#> fh.css

Testing

All tests pass for me locally. The purpose of this PR is to set up automatic testing on GitHub.