Think about welcome dialog vs theme vs CMS access

[microbit-matt-hillsdon]

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The welcome dialog now shows a video of the themed editor. This doesn't make much sense in the context of this repo and might be a barrier to others using this repo.

Perhaps deployments should have default flags? Then the welcome dialog could be behind a flag?

Needs some thought.

[microbit-matt-hillsdon]

One awkwardness in pulling this out is that we query the CMS to find the video.

So we'd need to do something like:

1. hardcode the video ID (maybe fine now we're less likely to change it?)
2. duplicate code to query the CMS
3. provide a CMS querying service to the deployment somehow (not worth this complexity)

I'll leave this for a while as it might become clear that (1) is a good choice over the next few motths.

Please comment on this issue if this is a barrier to your use of this project to help me prioritise.