Split external IPFS node type

[alanshaw]

I'd like to make it easier to use an external JS IPFS node. Could we split "External" node type into three:

1. External (go-ipfs)
2. External (js-ipfs)
3. External (custom)

Selecting 1 or 2 would hide/disable "IPFS API URL" and "Custom Local Gateway" fields (and set them to the default values for the node type you've selected).

relates to ipfs/js-ipfs#1563

[lidel]

The bigger overhaul of the backend picker is on the roadmap: #491
We probably should make this change as a part of that endeavor.

Some initial thoughts:

• 👍 for listing JS and GO options as equal
  • I imagine we could pre-populate list of backends with "localhost go-ipfs" and "localhost js-ipfs" with default localhost ports for each.
• Perhaps we could improve how "automatic mode" works.
  For example, if go-ipfs goes down extension retries it once and then falls back to trying other active backends on the list, including pre-populated JS one.