Allow one to use any non-word separator in accepted CAA types list

Use re.findall(r'\w+', str) instead of str.split(), it permits to handle
gracefully things like "front, ,back;;medium".
It is important since no format info is given in UI for now, simple text input.
Log final list of accepted CAA types to debug output.
If option "caa_approved_only" is set, skip non-approved images asap.
Allow only one image of each type, fixing PICARD-350

Number of images per type is limited to 1 by max_per_type variable.