Adds three functions:
`withStoredImages` iterates over a list of models.Alerts, extracting a stored image's data from storage, if available, and executing a user-provided function.
`withStoredImage` does this for an image attached to a specific alert.
`openImage` finds and opens an image file on disk.
Moves `store.Image` to `models.Image`
Simplifies `channels.ImageStore` interface and updates notifiers that use it to use the simpler methods.
Updates all pkg/alert/notifier/channels to use withStoredImage routines.
