Rename application controller identifier "lisk" to "app"

[nazarhussain]

Currently we are using a identifier lisk to specify the channel for application controller. Which in our case seems correct as we call our application lisk. But to align with custom application development, we should choose something more generic.

I suggest that we rather use app identifier for application controller. So who ever develop application with our sdk, can use this generic but more aligned identifier.

https://github.com/LiskHQ/lisk/blob/8f7ac1ce66e16616ecd38b7880d424630508b9ae/framework/src/controller/controller.js#L128

To clarify further, we are using above in a way channel.subscribe('lisk:ready', cb). In our case it seems correct but if someone develop another custom token and still use lisk identifier in code that will be confusing. So rather channel.subscribe('app:ready', cb) will be more clear and understandable.