[Sprint/HipsterHedgehog](bug): Fix/imagick upgrade migration

Closes #563 (closed) (needed to make an issue for the branch naming constraint)

Had to fix the issues I was having locally with Imagick to enable progress on other tasks that were reliant on this. With the help of @brianhatchet I have identified that the version installed is Imagick 3.4.0. Since Imagick 3.2.0 flattenImages() has been depreciated, I have replaced this with its recommended counterpart.