Synchronize with the Rocket v0.5-rc branch

[mentaljam]

This is a draft which main purpose is to prepare for coming Rocket changes which are in the v0.5-rc branch. Also the PR provides a new crate rocket_okapi_ui with functionality to embed Swagger UI into the application which is moved from the swagger_ui mod.

[TotalKrill]

I do like how the UI part is moved out! hopefully it would make it better to provide alternative openapi UIs later

[ralpha]

@mentaljam In GREsau/okapi master I have changed the UI part. Instead of putting it in a separate crate I opted for putting it under a feature flag.
I also added RapiDoc as an alternative to Swagger UI, also under a feature flag.
So this means that the UI is still part of the rocket-okapi crate but does not get included into the executable if the flag is not set.
Adding your own UI is still possible (it always was). Maybe in the future we can still separate the UI into its own crate.
If you have some feedback, let me know!