Fixed font size when specified in configuration

[LHBL2003]

If font_size is specified via configuration, then the fixed font size is used. If the font size is not specified or the value = 0, the previous dynamic behavior will still be executed.

Refers to the Issue: https://github.com/k01ek/netbox-qrcode/issues/11

[patschi]

@k01ek Apologies for tagging - I know it's rather rude. But anything speaking against merging this? This feature would be awesome.

[mhdask]

This probably needs to be updated, as the code has changed - However, I would also like this addition, and the opportunity to specify a different font_size per. object, e.g. different font_size used for cables vs devices

[cruse1977]

updated version in new pr includes this, thanks for your contribution.