Use '+' and 'X' for gallery/deselect all

in media library. You see the + button (means create gallery for now) if you are coming from a post. Create gallery not implemented yet.
Set featured image with media library

- Remove a lot of code from PostSettingsVC
Hide checkboxes when choosing featured image

- Users cannot create a gallery for a featured image, and they are probably not interested in deleting images at this point.

- Tapping an image does not go into edit mode, it sets the featured image.

- Don't show 'attached' media as we're focusing on choosing a featured image. The featured image is shown right before choosing on the settings screen.
Show gallery creation IFF the library is shown from a post
Load library items by scroll position

- Load images only if scrolling 'slowly'

- Load images for visible cells if we were scrolling fast, then decelerate.

- Follows what Reader does