update CI configs

[johnnychen94]

• replace travis/appveyor with github actions: GA allows more concurrency.
• update tagbot: https://discourse.julialang.org/t/ann-required-updates-to-tagbot-yml/49249
• add ssh key for Compathelper and tagbot: A github action cannot trigger another github action; adding ssh key enables CompatHelper and TagBot to trigger unit tests.

@zygmuntszpak you may need to uninstall appveyor hook when this gets merged.