Refactored code to use Subscription.add

[hirokiterashima]

Changes

• For components that had multiple observable subscriptions, I replaced them by adding the subscription to a shared Subscription via subscription.add(), and unsubcribed all at once in ngOnDestry().
• For components with single subscription, I left it the same.
• Fixed some subscriptions that were never unsubscribed.

Additional changes

In those affected files, I also cleaned up the code:

• removed unused dependencies in controllers
• removed unnecessary variable declarations in controllers
• removed unsubscribeAll() functions and instead call unsubscribe() directly
• removed EditComponentController because it's no longer used

Test

• Test all apps (student, teacher, author) and make sure that everything works as before. There is no new functionality, only refactored code.

Closes #66

[geoffreykwan]

Looks good.