Build/publish script refactor

[eritbh]

• Use Github Actions instead of Travis to keep tings centralized
• Make sure the build/verification portion of the script can be run locally without requiring Reddit credentials
• Use dart-sass on the command line as the standard for sass compilation (will allow the use of new language features faster, libsass is kinda slow) + document this in readme
  • Use new --no-charset dart-sass CLI flag to avoid having to strip that out manually in the build
  • Explore performance of --style=compressed dart-sass option compared to the currently used CSS compression API

[eritbh]

Another thing to address - we should probably just remove the validation step, it's unlikely to be helpful since Reddit's CSS rules are all we really care about and are stricter than w3c's most of the time.

I'm not sure how much of this issue is still relevant after #77 - need to go over this again at some point