Generate source URLs for wrapper links automatically

[amcclain]

Realizing that our github URL routes would be better if we auto-created them based on the versions of hoist-react and toolbox instead of relying on us to update (and then forget to update) the sourceUrls config.

Goal is to put the right branch/tag in the URLs - i.e. for production toolbox I am just updating the config to be:

{
  "hoistReact": "https://github.com/exhi/hoist-react/blob/v24.2.0",
  "toolbox": "https://github.com/exhi/toolbox/blob/v1.15.0"
}

The stems of the URLs aren't gonna change - we just need to update that last bit. I think we can look at the versions of hoist and toolbox and either
a) use develop in the URLs if they are snapshots
b) use the version tag if they are versioned builds

Then we should be able to remove the config entirely and not worry about updating.

[lbwexler]

Any chance that as part of this, we could create a token to "this example". Would be nice not to have to maintain that self-referential link everywhere.

[amcclain]

I looked a bit at options to get a reference to the current unbundled file name/path - it's an interesting question. It seemed like there was one option involving some node settings and variables that might be workable - I can play with it a bit more.

I'll also handle this ticket as I had created and have the plan for it.