set MIN_SCALE back to previous low value
requested to merge 702-no-useful-zoom-level-for-larger-documents-possible-no-zoom-farther-than-20 into master
Commit 27a4b3e5 changed minimum scale value from 5.4% to 20%, that's a sane value for normal sized documents (A4, letter,etc) but for large documents like comics is a too high value (preventing further zoom out).
So, set it back to its previous value.
Closes #702 (closed)