CE-1125 ImageReview refactoring - Prologue

This commit contains:
* Use Vignette as a thumbnailer
* The beginning of a code separation (hooks & db queries)
* Some documentation

Pinging @Grunny @macbre and @nmonterroso  due to Vignette changes
``ImageReviewDatabaseHelper`` operates on only one DB table. Can it connect to the database "internally" so we can skip passing database instance to each method?