subset fonts by script (e.g., alphabet)

- rewrite font generation script to subset font by script
- document scripts attribute
- move font-face declarations to separate CSS file
- update font-face declarations to reflect scripts selection
- only add fonts that are declared in stylesheet
- fix EPUB3 validation error with use of inline styles
- don't use bold in sidebar heading
- set font scaling on Namo Pubtree
- update worklog and comments