"Fork" button doesn't reset its state on fork error

[lbogdan]

When there's an error forking a (server) sandbox (for example because the server sandboxes limit was reached), the "Fork" button doesn't reset its state and remains in the "forking" state:

[lbogdan]

@ValentinH Can you maybe take a look?

[ValentinH]

The disabled/loading state is shown when store.editor.isForkingSandbox is true. Thus, we probably need to reset it to false on error.

I'll have a look later today when I'm on the computer.