Reactify route-based errors

[cjcenizal]

We currently have three errors which are surfaced via individual Angular routes:

• AllowExplicitIndex ([errors/multi.allow_explicit_index] move error handling to browser #14184)
• AutoCreateIndex ([savedObjects] Use index template #14271)
• UrlOverflow (Implement/better warning for url length #6773)

We'll need to convert this code to React. As part of this process, we should consider removing the need for specialized routes altogether since the UX of allowing a user to navigate to an error page when there's no error doesn't make sense. It also seems unhelpful to navigate out of the context which caused the error. As an alternative we could display a large modal. We should also ask our copywriting team to take a look over the copy while we're at it.

[pgayvallet]

Closing. Url overflow error page has been migrated to KP, the two others are no longer a thing.