Enable ability to mock endpoints in development mode

[kevinansfield]

no issue

• moves existing mirage config into the testConfig() function to retain full mocks during testing
• configure mirage to passthrough all requests when in development mode

Our existing mirage config that we use for testing was set up as the default/development config which meant that we had to explicitly disable it when in development mode - this has the unfortunate side-effect of removing the ability mock endpoints when developing new features.

This PR moves everything we currently have into the "test" config and sets up a new mirage development config where endpoints can be overridden with mocks and any requests that aren't handled by a mock will be passed through to the real API.

[jaswilli]

I don't love the idea of this being turned on by default in development mode. It seems like a potential source of headaches because we're now subject to bugs and/or strange behavior in this dependency.

Also, having this on by default opens us up to all sorts of "oops, this was being mocked in development and we just released to production" defects.

Putting the configuration in place so that it can be turned on for local development seems reasonable but always having it on makes me uneasy.

[kevinansfield]

Good points, this was cherry-picked over from a branch where I needed to get this working so the default-on was left in. I'll update to keep mirage disabled in development but leave the setup as-is here so that it's easy to enable when required.