Replace isomorphic-fetch

[znarf]

isomorphic-fetch is not actively maintained and is using old versions of node-fetch and whatwg-fetch:

"dependencies": {
    "node-fetch": "^1.0.1",
    "whatwg-fetch": ">=0.10.0"
  },

Potential maintained replacements are:

• cross-fetch: https://github.com/lquixada/cross-fetch
• isomorphic-unfetch: https://github.com/developit/unfetch/tree/master/packages/isomorphic-unfetch

Also, with cross-fetch and isomorphic-unfetch the 'polyfilling' is optional., which is a better approach.

We should also replace our calls from node-fetch to cross-fetch/isomorphic-unfetch and remove node-fetch as a dependency.

[znarf]

That was implemented as part of #528