feat(insights): changes to eap toggle for ea

[DominikB2014]

1. Make eap the default if the flag is enabled
2. Remove eap toggle on all pages except backend overview (this is because the backend overview page has a new layout, whereas the other pages are just a dataset swap primarily)
3. Update eap toggle to say "switch to old ui" and "switch to new ui"
4. useInsightsEap is now updated to always prioritize the useEap query param (we'll use this for testing between datasets internally, but customers aren't expected to use this). It also checks against the new backend ui being enabled
5. Ditch useSyncedLocalStorageState, it seems to be a little flakey, instead we'll just update the query params when the user switches between the "old ui" and "new ui"
6. Don't show the eap toggle, if the next.js toggle is active

[narsaynorath]

lgtm! 👍