Flipper UI crashes if feature key contains a '/'

[alainravet]

How to reproduce:

Step 1 : create a key in the console

  bin/rails c
  > Flipper.enabled? 'a/b'
    => false
  > Flipper.enable 'a/b'
    => true
  > Flipper.enabled? 'a/b'
    => true

Step 2 : try and edit the feature in flipper ui

 open https://mydomain.com/features/features
 click on the 'a/b' key
 -> https://mydomain.com/features/features/a/b
                                           ^^^

result (in the browser)

  Flipper::UI::Actions::Gate does not support request method "get"

[jnunemaker]

I think I've always imagined that feature keys would be string/number, but I don't think it is enforced. I'll take a stab at making this work. If someone else gets itchy first, feel free.