Switch from Travis CI to GitHub Actions

[KitsuneRal]

Travis CI left open source projects in limbo after publishing a blog post in November and never updating since then, with a support ticket closed with the "waiting for management decision" wording. The easiest is to fall back to GitHub Actions that in the meantime matured and gained enough features to be considered ready for our intents and purposes.
Things to cover:

• Platforms: Linux, macOS
• Compilers: GCC, LLVM/Clang
• With and without E2EE
• With and without making update-api
• Run Quotest
• Add Valgrind to Quotest invocation
• (optionally) ...in parallel (not done on Travis CI) (see Enable parallel runs of quotest #435)

The qmake workflow may be introduced in the stable branch if the number of changes will be large (which is not anticipated now).