* Refactor NotFound component

- shared NotFound becomes NotFoundPrompt - returns only an EuiEmptyPrompt, and individual products/plugins are in charge of their own layout, rather than NotFound doing a bunch of arduous switch handling (also closer to how errorConnecting is a component set per-plugin)

- This is both due to the recent page template refactor and the fact that WS has extra complex logic of needing to switch between its kibana layout and personal dashboard layout

- logos are still hosted in shared/ since they need extra custom CSS to work correctly sizing wise and in dark mode. I renamed its folder from `assets`->`logos` for extra clarity

* [AS] Update current AS routers using NotFound

+ update EngineRouter to use NotFound

* [WS] Update app router

- Handle errorConnecting at the topmost level, instead of in WorkplaceSearchConfigured (to simplify various logic/expectations & match App Search)

- Simplify isOrganization check to use `useRouteMatch` instead of a regex

- Use new NotFound component
- Add NotFound component for the personal dashboard router

* [WS] Improve Source 404 UX

- Add NotFound to SourceRouter + add breadcrumbs for organization views

- When an actual source ID 404s, fix blanket redirect to a dashboard aware redirect - personal dashboard 404s should send the user back to personal sources, not organization sources
+ add a flash message error (similar to how App Search behaves for engine 404s)
+ harden error status checks (gracefully allow for non-http errors to fall back flashAPIErrors

* [WS] Improve Settings 404 UX

- This was the only remaining WS route I found that either did not have a 404 or a fallback to some overview page, so I tweaked the redirect order for a graceful redirect (vs a blank page)

* Fix settings router test

* Move away from custom product logos to OOTB Enterprise Search logo

Keeping it simple, etc. RIP in peace fancy logos

* [PR feedback] toContain over stringContaining