fix fall-back on custom font declaration

Both ways work in the end, but the fall-back happening here is not from one
font family to another but from a possible local installation of the same font
to using a webfont version from the app. The CSS handling this fall-back
should be the font-face declaration, not the family selector.
add webfont resource files for Libertine font family
add config option and styles for Libertine font family
avoid over-riding font family preference if the font has good glyph support
reduce font size boost for Hebrew from 1.5em to 1.2em

The 150% size increase is excessive and makes usage impractical with
several columns open. 1.2 is a more usable default value and works out
so that Hebrew versions take up about the same about of vertical space
as other languages for the same verse range.