Add CI to enforce linting

[jwngr]

[mohawk2]

You sort of have that now; the CI is trying to deploy, but failing the build (eg https://semaphoreci.com/jwngr/sdow/branches/master/builds/123):

Treating warnings as errors because process.env.CI = true.
Most CI servers set it automatically.

Failed to compile.

src/components/ErrorMessage.js
  Line 37:1:  Assign arrow function to a variable before exporting as module default  import/no-anonymous-default-export
[etc]