[develop] Github Actions for Community PRs

[scottarnette]

Change Description

Problems:

• Travis limits us to a certain amount of jobs at a time for the entire github org.
• It is also limited on total time a job can run.
• The hardware available is limited and slow.

Solution:

• Replace Travis with github actions so that we can triple the amount of concurrent jobs.
• Github actions job timeouts are much longer than Travis.
• The hardware is far more powerful in github actions.

See:
https://github.com/scottarnette/eos/pull/3 | Forked PR. This PR would merge this branch into my fork of EOS.

https://github.com/scottarnette/eos/runs/381994338 | Actions build using commit 77d6721 of this branch on my fork.

https://github.com/EOSIO/eos/commit/51dd4a8b9a44874cf3640a2926d8ccd309b3cce0/checks?check_suite_id=390896314 | Actions build of this PR. Because this PR is not a fork, we do not run build steps on Actions.

NOTE: If all steps in Actions are skipped, the build is marked as failed. The "Start Job" step exists to fix this issue. All build steps are triggered from that step IF the PR is forked.

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions