Remove dependencies on isomorphic-fetch

https://caniuse.com/#search=fetch

While it was a new feature at the time and I can kind of see why we polyfilled it, I think we should drop isomorphic-fetch from our bundle and rely on the native implementation in browsers. At this point it's just one more dependency to maintain and dead weight in our bundle as we are in no way compatible with anything that doesn't have fetch at this point.

Our nightmare and mocha tests will run fine but we might need to augment the stripes-connect test. fetch-mock or similar will do that handily without much fuss.

Packages where we currently depend on this and will no longer need to maintain it:

• stripes-core

• stripes-connect

• ui-scan

• ui-calendar

• ui-requests

• ui-circulation