poc/feat: slotless page templates

[kaljarv]

WHY:

Fixes #375

What has been changed (if possible, add screenshots, gifs, etc. )

This PR is a PoC draft. Go to onto the /_test route to see the slotless layout in action. Note that it doesn't yet attempt to fully recreate the actual layouts.

Some remarks:

• The context used is just for testing and does not adhere to the future VoterContexts etc. (see: poc/refactor: data api and state management #573)
• Instead of a PageConfig component, pages could just directly access the relevant context methods.
• We might need something more involved instead of a HeaderActions component if we wanted to allow Svelte layouts to add items to the header that would be there on all subpages, i.e. /foo/+layout.svelte could add a Foo button that would be on all subpages, and /foo/bar/+page.svelte could add a Bar button so that both would be shown.

Check off each of the following tasks as they are completed

• I have reviewed the changes myself in this PR. Please check the self-review document
• I have added or edited unit tests.
• I have run the unit tests successfully.
• I have tested this change on my own device.
• I have tested this change on other devices (Using Browserstack is recommended).
• I have tested my changes using the WAVE extension
• I have added documentation where necessary.
• Is there an existing issue linked to this PR?

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