Allow working with a new (unsaved) entity

Or … better yet: what if we allow navigating to /xb without any entity, which then would load the XB UI without any request to /xb/api/v0/layout/{entity_type}/{entity}.

I'll get a draft MR going with the server-side bits.

Quickly hacked together an initial MR, seems to at least kinda work? Now just needs JS updates to finish booting :)

Oh and this of course would still show the navigation bar with the UI:

Created 📌 Add permission for "Use Experience Builder" Active , and postponed on this

Just discussed this with @effulgentsia and @penyaskito. We agreed that create access for the xb_page content entity type would be a reasonable first step until 📌 Allow XB to be used on all node types Active happens.

This blocks:

• ✨ Move entity type and ID from base path and into routing parameters Active
• 📌 Add generic create entity support Active

I don't think this needs to block the beta. It would be very nice to have though for the beta since this would make testing XB outside of Drupal CMS / other pre-defined setup much easier.