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.
If an image token is present in an alert instance, the email notifier will attempt to find a public URL for the image token. If found, it will add that to the email as the `ImageLink` field. If only local file data is available, the notifier will attach the file to the outgoing email using the `EmbeddedImage` field.
This change extracts screenshot data from alert messages via a private annotation `__alertScreenshotToken__` and attaches a URL to a Slack message or uploads the data to an image upload endpoint if needed.
This change also implements a few foundational functions for use in other notifiers.
