Make sure we're using the new account system everywhere

[koke]

Some parts of the code still use [WordPressComApi sharedApi].username to get the WordPress.com account. Migrate everything to the new WPAccount system.

[koke]

WordPressComApi was designed to be a singleton, so that needs a bit of refactoring as well.

Note objects should belong to a specific account, as well as ReaderPost. When account is deleted (sign out), those objects should be removed.

[koke]

We'll leave the refactor for 3.7.1, but make sure WordPressComApi and WPAccount "talk" to each other and are aware of sign in/out, and password changes

[koke]

I think this was closed by mistake

[sendhil]

Ahhh, we ended up closing a few issues by accident when we merged in from the other repo with a different ordering system.

[astralbodies]

@koke I noticed that [WPAccount defaultWordPressComAccount] uses the main context - we need to be mindful of where we're using the WPAccount object after it's fetched or provide a context parameter

[pivotal-rebecca]

FYI, not sure how much the previous refactoring work has been looked at, but this was already (at least partially) completed here: 210a806

[sendhil]

Here's an instance where this was missed:
https://github.com/wordpress-mobile/WordPress-iOS/blob/develop/WordPress/Classes/SettingsViewController.m#L191

[koke]

Taking over this as part of #538