Add backend code for processing and storing image recommendation user feedback submitted via the VisualEditor API's plugin mechanism.
* Create a `growthexperiments_image_submission` table, similar to the existing `growthexperiments_link_submission` table, but with the appropriate data fields instead of offset/length/target/feedback, and a DAO class for interacting with the table.
* Create an image recommendation submission handler service, similar to the AddLinkSubmissionHandler and LinkSubmissionRecorder classes (there probably wasn't much point in those two being separate classes), maybe with a shared SubmissionHandler interface.
* Add some mechanism to the VisualEditorApiVisualEditorEditPostSave hook handler to select the appropriate submission handler based on the plugin data.
(Similar Add Link tasks: T266446, T269657)