Try to fetch guide remotely if not found locally

[oskarhane]

When trying to play a local guide: :play reco and it isn't found, an attempt to find it on the url browser.remote_content_hostname_whitelist + name is made.

browser.remote_content_hostname_whitelist is a server configuration list of hosts (or urls:s) that the browser can fetch remote content from.

The default value for browser.remote_content_hostname_whitelist is localhost,guides.neo4j.com, so the above play command would resolve and display the reco guide hosted on https://localhost/reco, then on http://localhost/reco, then on https://guides.neo4j.com/reco and finally on http://guides.neo4j.com/reco.
It stops after the first successful fetch.

The steps followed are:

• Try locally. If found, display it
• Try remotely, by appending the guide name to each of server config list of browser.remote_content_hostname_whitelist
• Iterate over whitelist and if guide is found, display it and stop trying to fetch guide from the rest of the list
• If error
  • show 'Not found' frame

Some updates to on how we use isomorphic-fetch are added as well, so errors are thrown when we hit a non 200 status from a request.

[pe4cey]

@oskarhane it seems that it only works when connected to the neo4j server.
To recreate: :server disconnect :play arrows

[oskarhane]

@pe4cey Updated

[oskarhane]

It'd be great to have your view on this feature and implementation @akollegger.

Should the config be moved to server config to have it centralized?
Suggestions for a better name on the configuration option would be appreciated. 🎈

[akollegger]

Well, wdyt about two things:

1. use the server whitelist as a "search path" for remote content
   2a. making the browser.* configuration served up by the server as static browser content
   2b. holding onto this PR until the server-side changes have been made for 2a

[oskarhane]

I like the idea of using the whitelist as a search path @akollegger.
I'm not sure I understand what you mean by 2a.

Also, there might be the case where the browser isn't connected to a server (we might not even know the server url) and what should happen in that case? That's why I made it a client side config in the first place.
Maybe just a fallback til the guides url in that case?

[oskarhane]

This is now ready for review again @pe4cey (and more comments if you have any @akollegger).
Since I've changed a lot I've updated the expected behavior in the PR:s description.

[oskarhane]

Rebased @pe4cey