Fix: position of empty view to avoid message disappearing when keyboard shows up

Description

Changing the emptyViewController to manipulate your size and position using constraints to the View, instead of using frame manipulation.
Changing the bottom constraint of the emptyViewController to follow the Keyboard height when the same appears.

Blog Post - With Images	Blog Post - Without Images	Media - With Images	Media - Without Images

Gray space before	Gray space after

This can be tested by cloning my fork(and branch) and changing the WordPress-iOS Podfile to use the Pod locally:

pod 'WPMediaPicker', :path => '...'

Run the app
Switch to a site that has no media uploaded
Open Blog Post and add an IMAGE block
Make sure that the custom empty view is displayed correctly
Opens the Keyboard
Make sure that the message No media matching your search is displayed correctly