Fix support for `initUrls` in `startData`

[nf-s]

Fix support for initUrls in startData

Fixes https://github.com/TerriaJS/terriajs/issues/6403

You are supposed to be able to do this in #start=

For example

{
  "version": "8.0.0",
  "initSources": [
    "http://gist.githubusercontent.com/nf-s/67e44309245f3caca1cee5cdf2eee3eb/raw/75c50871fd972f8d6f35039c6e5278f3f7437a69/colorscale-minmax.json"
  ]
}

But it isn't working

Test me

• Before http://ci.terria.io/main/#clean&start={"version":"8.0.0","initSources":["http://gist.githubusercontent.com/nf-s/67e44309245f3caca1cee5cdf2eee3eb/raw/75c50871fd972f8d6f35039c6e5278f3f7437a69/colorscale-minmax.json"]}
• After http://ci.terria.io/fix-initurl-startdata/#clean&start={"version":"8.0.0","initSources":["http://gist.githubusercontent.com/nf-s/67e44309245f3caca1cee5cdf2eee3eb/raw/75c50871fd972f8d6f35039c6e5278f3f7437a69/colorscale-minmax.json"]}

Checklist

• [x] There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• [ ] I've updated relevant documentation in doc/.
• [x] I've updated CHANGES.md with what I changed.
• [x] I've provided instructions in the PR description on how to test this PR.