Allow reset subscription cursor to the END

[SergKam]

Allow reset subscription cursor to the END
User can already create a new subscription using "END" as a cursor position.
Can we allow users to use END cursors the same way as we use BEGIN cursors?
It would be very useful for debugging, firefighting and makes less outdated subscriptions (people don't need to create a new subscription to start from the last current cursor )
zalando-nakadi/nakadi-ui#40