AragonApp: provide initialize() function for subclasses to chain initialization blocks

[sohkai]

We could provide a default initialize() function as part of AragonApp that just calls initialized().

This would allow us to recommend all AragonApp subclasses to include an initialize() block that always invokes super.initialize() immediately (this is very similar to how React does component construction), greatly simplifying how contracts subclassed more than one level deep (e.g. Agent, fundraising's Pool) can define custom initialization behaviour.

From React's documentation:

When implementing the constructor for a React.Component subclass, you should call super(props) before any other statement.

cc @izqui @facuspagnuolo @osarrouy