Defaults image attachment size to "full"

[SergioEstevao]

Fixes #830

To test:

• Open the demo app
• Add an image
• Switch to HTML
• Check if the full attribute is applied

[jleandroperez]

Works as advertised!!

[designsimply]

I tested with iPhone 6S, iOS 11.1.2, WPiOS 8.9.0.20171122 and I was unable to see the class="size-full" attribute.

Steps to reproduce:

1. Go to WPiOS > Blog Posts > + and add a title.
2. Insert an image from the device's media.
3. View the HTML to see if class="size-full" is present.
4. Go to WPiOS > Blog Posts > + and add a title.
5. Insert an image from the blog's media library.
6. View the HTML to see if class="size-full" is present.

Result: when the image was inserted from the device I saw class="wp-image=11507 and when the image was inserted from the media library there were no attributes other than src.

[diegoreymendez]

@designsimply - Can you move this info to the issue here? (as opposed to the PR)

It will help ensure the feedback is properly tracked and there's an explanation if we need to reopen the issue.

Thank you!

[SergioEstevao]

Humm by default on the main app I don't think we are setting any size attribute. Should the size-full be the default @designsimply ?