Fix CI

[bcomnes]

Getting started on some of the chores we discussed previously, First up, fixing DDE's CI system.

Appveyor + electron builder seems to be having some issues, so rather than fix those, I proposed we switch to GitHub actions, a CI system native to GitHub. Also, since GitHub is Microsoft now, it supports very decent windows build environments (unlike Travis last we checked).

Here is the plan:

• Set up actions for windows, Mac and Windows.
• Turn off travis and appveyor.
• Ensure our release process works the same or closely resembles our previous process.