Remove use of DefinePlugin, switch to VUE_APP env vars

[jjnesbitt]

This uses a more standard way of injecting env vars, and removes most of the complexity from vue.config.js. I've also created a .env.default file, as a template that describes the new env vars.

Let me know what you guys think.

[waxlamp]

This is awesome. I have one idea for this, and I would like to know what you think: should we have a new module, environment.ts or something, which could just export all of the VUE_APP_* variables we make use of? It would be a nice way to express the "interface" between the client app and the environment, and it would also make clear what the default values are that the app is using, all in one place.

CI is currently choking on the client testing; I'll take a look at that.

[waxlamp]

Looks awesome. Just the one small change and this is good to merge (as soon as we figure out the testing issue).

[JackWilb]

Looks like the client comes up during testing and on my local. However, running yarn test:client:up on my local fails because of one command, curl -s -I --max-time 0.5 http://localhost:8080/api/workspaces 404s.

[waxlamp]

Looks like the client comes up during testing and on my local. However, running yarn test:client:up on my local fails because of one command, curl -s -I --max-time 0.5 http://localhost:8080/api/workspaces 404s.

Ah I figured it out: the point of this PR is that it detaches the server from the client altogether. So it's no longer viable to use localhost:8080/api to probe whether the client build has succeeded; however, we can do the same thing by just curl'ing localhost:8080 instead. See 8e3d7a5.

[JackWilb]

Looks good!