Use pylibdmtx for datamatrix generation and fix datamatrix consistency
This MR
- Removes pyStrich as a dependency
- Instead uses pylibdmtx for datamatrix generation
- Updates the barcode example for the exam editor
- Fixes the font size of the datamatrix on the first page
- Makes the exam preview consistent with the barcode example
Closes #319 (closed) and #323 (closed).