Add an appStatus to app controllers and scope

[w33ble]

This PR adds an appStatus to the controller and scopes for Discover, Visualize and Dashboard. It uses this status to track whether or not the app is "dirty", that is, if it has been modified since it was last saved.

• Add state monitoring module
• Refactor Discover and Visualize into directives
• Add dirty checking to apps based on appState, and in the case of Visualize, editableVis as well
• Fix config rendering, firing postLink after injecting in the DOM so that directives with a require will function

[epixa]

Use es6 modules: export default monitorStateChanges;

[epixa]

Can you get some tests in for these changes wherever it is practical?

[cjcenizal]

Nice cleanup job!

[cjcenizal]

I left a few comments, but I also have a suggestion for making the interface for this new service more intuitive.

How about if it was used this way:

const StateChangeMonitor = require('ui/state_management/state_change_monitor');

const stateChangeMonitor = new StateChangeMonitor($state, stateDefaults);

stateChangeMonitor.onStateChange(isDirty => {
  $appStatus.isDirty = isDirty;
});

$scope.$on('$destroy', () => {
  stateChangeMonitor.destroy();
});

I find this easier to follow for a few reasons:

1. There's no possibility of having shared state between controllers, because each owns its own instance of stateChangeMonitor.
2. Passing the constructor references to a default state and a changing state implies that it is primarily concerned about those two things, which implies its job is to do comparisons.
3. The event-handler-registration method is explicit, and supports the idea that its job is to do comparisons.
4. Calling destroy is also very explicit.

The only thing I think I'm not anticipating is the use case where you force the dirtyChecker to return true if it's already dirty. If you like my idea then maybe we can discuss the requirement in that case and figure out how to work it in?

[tylersmalley]

This looks great! Reviewed the code and implemented a test in Editor.

LGTM

[cjcenizal]

LGTM once w33ble#7 gets merged!