refs https://github.com/TryGhost/Admin/pull/2209
- `miragejs` has been extracted to a framework-independent library, the re-exports of `miragejs` elements in `ember-cli-mirage` have been deprecated making our test logs very noisy
- added `miragejs` as a top-level dependency
- updated all relevant imports to pull from `miragejs` instead of `ember-cli-mirage`
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