Add mechanism to refresh the token

[czerwonkabartosz]

Now the token is valid for 24 hours, so if the user is on the site for more than 24 hours and then sends a request the server will reject the request because of a invalid token.

The best option is to create a TokenManager (name to rethinking) in https://github.com/ckeditor/ckeditor-cloudservices-core that will download new token every X time from URL.

Pseudocode:

class TokenManager {
	constructor( tokenUrl, time = 3600 ) {
		// ... emit event if new token received
	}
}

class UploadGateway {
	constructor( tokenManager ) {
		tokenManager.on( 'changed:token', () => {
			// ....
		} );
	}
}

const tokenManager = new TokenManager( 'url' );
const uploadGateway = new UploadGateway( tokenManager );

[czerwonkabartosz]

As @pjasiun mentioned, we TokenManager should be called Token.

I would call it just Token:

const token = new Token( 'url' );
const uploadGateway = new UploadGateway( token );
token.value // the value of the token, also `change:value`, but note that it will be used only internally, for the external user it will be just a `token`.