Better errors on failure to create a project

[lwjameson]

Summary

Currently when a project fails to create there is little user feedback and you often end up at a 500 error page. Although it was discusses, and I experimented with doing validation, it proved too slow and clunky to actually make it work. As an alternative, this PR implements detailed error messages on the BottomBar and keeps the user on the create page so that may address the issue and try saving again.

Screenshots

Notes

• This PR adds nanostores to the admin-client to share the error state with the BottomBar
• There is also a small fix to try and address multiple page loads on the Projects page.
• An endpoint to validate a project slug does not exist. Although it is not currently used I decided to retain it.
• There are other areas where the BottomBar is used that could also display errors, but they are not yet implemented.
• The nanostore could also be used on pages that do not have the bottom bar, but where backend errors could occur. We may want to do this in the future.

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