Make requests cancellable

[davidrapin]

Using the proposed /api/cancel endpoint, make all API calls cancellable.
see https://github.com/Linkurious/linkurious-server/issues/779

How

Add a cancellable boolean option to all option object of all requests
When options.cancellable is true:

• generate a random cancellationToken and add it to the request parameters
• add a cancellation handler in the request response promise chain (.then(onResolve, onFailure, onCancel))
• if the promise is cancelled, use the generated cancellationToken to cancel the server-side query using the /api/cancel endoint