closes https://github.com/TryGhost/Team/issues/411
- adds "Create snippet" icon to the editor toolbar
- uses the same link input component design for specifying snippet titles
- snippets are loaded in the background when the editor is accessed
- snippets are listed at the bottom of the card menus of the + and / menus
- clicking a snippet inserts the snippet's contents in place of the current blank section
5606b9c068