refs https://github.com/TryGhost/Team/issues/1217
- adds `tenor` service that acts as a coordinator for the Tenor API similar to the `unsplash` service
- adds `<GhTenor>` component that renders an image search and select modal using the `tenor` service
- swapped the gifs card over to use the `tenor` image selector so it opens the tenor modal instead of the unsplash modal
