* Send new annotation containing image url
* Use new image TokenProvider with TokenStore
New abstraction GetImage no longer needs to support parsing both token and
url from annotations, as remote AM will use the new URLProvider. Instead, we
use the new generic TokenProvider and give it a TokenStore backed by the
grafana database.
That means we revert back to always using token simplifying code and security
considerations.
* Upgrade grafana/alerting to merged commit SHA
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.
