Apply all `InitSources` sequentially instead of asynchronously

[nf-s]

Apply all InitSources sequentially instead of asynchronously

Fixes #5949

InitSources should be applied on top of each other - in order specified by terria.initSources array.

I have added an init-sources doc - https://github.com/TerriaJS/terriajs/blob/abac123141f60541fb5b383e60355c2f26ec7a5b/doc/contributing/init-sources.md

For example - these should be applied in order

• config.initializationUrls
  • these are added in terria.start() -> terria.setupInitializationUrls()
• updateApplicatonUrl
  • interpretHash
    • init fragments - eg #simple or #https://some.url/config.json
    • #start
    • #share
  • /catalog/ routes
  • /story/ routes

When InitSources are applied (see terria.applyInitData()) there are two async tasks

• loadModelStratum
• Loading models and adding them to the workbench

Depending on these tasks, InitSources may be applied out of order, at random times - and may get interlaced

This PR makes sure InitSources are applied in order

Test me

The following URL has

• configUrl
  • https://gist.githubusercontent.com/nf-s/167f4749a4911288c54641589f417ecd/raw/55fd4f27c28dfe4636e119d84b2b1503117624a7/geo-rapp-fragment-path.json
• This add the following initializationUrl
  • https://terria-catalogs-public.storage.googleapis.com/geo-rapp/dev.json
    • Which set workbench: ["tN2nrg", "LfaYaM", "idvaI5"]
• Init fragment https://terria-catalogs-public.storage.googleapis.com/geo-rapp/dev/australia.json
  • Which should override workbench: ["nrm-regions-2017"]

Before - http://ci.terria.io/main/#configUrl=https://gist.githubusercontent.com/nf-s/167f4749a4911288c54641589f417ecd/raw/55fd4f27c28dfe4636e119d84b2b1503117624a7/geo-rapp-fragment-path.json&https://terria-catalogs-public.storage.googleapis.com/geo-rapp/dev/australia.json

After - http://ci.terria.io/sync-init-source/#configUrl=https://gist.githubusercontent.com/nf-s/167f4749a4911288c54641589f417ecd/raw/55fd4f27c28dfe4636e119d84b2b1503117624a7/geo-rapp-fragment-path.json&https://terria-catalogs-public.storage.googleapis.com/geo-rapp/dev/australia.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)
• [x] 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.