Toolbar upgrades and fixes.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/370) and [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/292).

Still has room for improvement, especially in the interface between `WPEditorViewController` and `WPEditorToolbarView` (ie: the code ownership is still not 100% right, and the delegate can probably be removed or improved).

There's also room for improvement on the custom toolbar items (no new issues should've been introduced now though), although we're not using them now so they shouldn't break anything for the time being.

This can be tested in WPiOS branch `issue/editor-370-link-button-6plus`.

/cc @bummytime 
[Now we do](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/380).