Remove simulated pull to refresh from most views

[mikejohnstn]

As discussed in #355, the simulated pull to refresh is distracting. This pull removes all of them except those in web views, which are handled differently.

This change led to a simplification of the API for syncing items in tables. I decoupled regular background syncing and user-initiated syncing, with the former now called via a simple syncItems. Manual syncing (e.g. via a pull to refresh) allows subclasses to do extra stuff if they need to.

I intended to preserve all "extra stuff" that's done by a manual refresh (including post pruning and the syncing of extra metadata), but a careful review is in order since this affects so many views.

Before/after:

[mikejohnstn]

As per @koke's comment #355 (comment) I'll also update this to use UIRefreshControl. Good call, though we won't be able to rip EGO yet since it's still buried in web views and I'd rather not touch those with this pull.

@koke could you put comments about additions etc. in the pull request next time instead of the issue? It aggregates a nice conversation about the implementation details.

[mikejohnstn]

[mikejohnstn]

With these changes, after first signing in, I believe you're left staring at a blank screen for too long. I've added #376 to consider improving this case.

[astralbodies]

@mikejohnstn I'm looking through WPTableViewController, and I see the new method syncItemsViaUserInteractionWithSuccess:success:failure: but I don't see that the method is actually called anywhere. Can you confirm?