CI: detect missing glyphs in language file

[mmilata]

Currently, adding new glyphs requires manually regenerating font bitmap files. If a language JSON is updated with a string that uses new glyph then Trezor users will see �.

There are several approaches:

• simple python script to compare the character whitelist that gen_font.py uses against set of chars used in language JSON
• detect this in the renderer and panic!() in debug builds
• CI job regenerates bitmaps then compares them with bitmaps in git
  • disadvantages: TTF of some fonts is in private repo, may possibly break on freetype updates
  • currently the characters are manually whitelisted, we'd also need to generate these from langJSONs

Related to: #992