Add support for WP core's `custom-logo` feature

[christophherr]

Beans, being the progressive theme that it is, has been using a custom logo_uploader in the Customizer before WordPress core added the custom-logo theme support in 4.5.

If a child theme adds theme support for custom-logo, Beans is showing two image uploaders in close proximity and will only display the "beans_logo" (unless the child theme does some additional work).

It would be possible to add a backward compatible workaround allowing both features existence. After a discussion with @hellofromtonya, it seems to make more sense to "port over/merge" existing "beans_logos" to the Core feature, remove the custom Beans solution and add theme support for custom-logo by default.

[hellofromtonya]

@christophherr Let's target this one for v1.6 release. We want to give some thought, discussion, and a lot of testing to ensure it moves everyone forward without breaking anything.

I can't see why we'd want to have a beans_logo Customizer field these days when we could use the default custom-logo that is built into Customizer.

Therefore, I do think it's better to replace it out and use the default. That means we'll need to write code that does the swap upon upgrading or activation.

[christophherr]

First pass at adding Core's custom-logo feature and 'migrating' existing beans_logos. https://github.com/Getbeans/Beans/compare/development...christophherr:custom-logo

• Added theme support for custom-logo. Child theme's can 're-declare'/overwrite with dimensions, etc...
• Removed the beans_logo_image section/control from the Customizer.
• Changed the header fragment to use Core functions to output the custom-logo and added the existing Beans CSS class through a filter. The advantage is that WP automatically outputs all responsive image sizes.
• Added a 'migration' function that basically works but the image/logo is not cropped to the correct size yet. (In other words, 'Change logo' in the Customizer has to be used to crop the image.)