feat(server): introduce a cors handler

Introduce a handler that lives under the gorilla mux and adds the correct
headers based on the request and always returns 200 OK when there OPTIONS is
called on a URL.

This fixes the ability to DELETE from the dashboard on peer X when peer Y is
the leader. As a side effect it reveals some bugs in the dashboard though
notably:

- Due to the RTT immediatly refreshing the dashboard doesn't work and
 deleted keys are still there

- For some reason PUTS from peer X are creating directories and not
 keys.