Add storyboards and xibs to the resource_bundle

[jtreanor]

In #67, as part of converting to a static framework, I moved images and assets to WordPressAuthenticatorResources.bundle. .storyboard and .xib files remained in the main bundle.

This mostly worked fine for WordPress-iOS, but has proved to be problematic in some cases there and in all cases for WooCommerce-iOS. Image references in storyboards were not resoving, with errors like this:

2018-12-11 09:21:31.650079-0800 WordPress[91387:1155878] Could not load the "beveled-blue-button-down" image referenced from a nib in the bundle with identifier "org.cocoapods.WordPressAuthenticator"

Fixes #38.

The solution I have implemented is as follows:

• Move .xib and .storyboard files into the resource_bundle. This means images will be loaded from the same bundle and it will find them correctly.
• Rename WordPressAuthenticatorResources.bundle to WordPressAuthenticator.bundle. This is a surprising quirk of CocoaPods to ensure they are all in the same target. More info here.
• Related code changes to load the right bundle. It's simpler now.

To test:

• Point WordPress-iOS to this branch (I have made a branch here to save some effort).
  • Smoke test the login flow, making sure images are loaded correctly.
• Point WooCommerce-iOS to this branch (I have made a branch here to save some effort).
  • Smoke test the login flow, making sure images are loaded correctly.

[mindgraffiti]

This worked perfectly for Woo. Thank you @jtreanor!

[jtreanor]

That's great, thanks @mindgraffiti!

I have tested WordPress-iOS quite thoroughly with this and I'm satisfied that all is well now so I'm going to go ahead with the merge.