Features could be listed and added through the api, now also removed.
This was needed in the case of gitlab.com as the number of gates that
were ever used just grows and cleaning up is hard.
This allows to enable/disable a feature flag for a given user, or a
given Flipper group (must be declared statically in the `flipper.rb`
initializer beforehand).
Signed-off-by: Rémy Coutable <remy@rymai.me>
