Add backend code for processing and storing image recommendation user feedback submitted via the VisualEditor API's plugin mechanism.
* 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. (See also {T289217}.)
* Declare the new Special:Log event subtype to use for recording image recommendations. The i18n messages should be placeholders for now.
* 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)
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This is pending further discussion and might not be needed:
* 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.