Call to DELETE /keys/ uses not-fully-escaped vapid_key string

[thexeos]

While testing the dev-dashboard, I used a Base64 (not URL friendly) public key to create a new application. When attempting to delete it, I got MessagesAPIError. It turned out the vapid_key was passed literally without encoding the forward slash.

The fix is to use quote(instance.vapid_key, safe='') to force all special characters to get URL encoded. #237