try params key access by sym and by string
export from Url instance and simplify loading params

When exporting a font, the font was
loaded from the existing file in the tmp directory.
additionally, the loading of the font parameters
was complex and tried to find out whether to read
the file and from which one.

instead this whole stateful complexity is simplified:
* load the Url from the db when exporting (as we
  provide the font hash anyway from the client)
* always load the params from the original file.
  the parameters are merged with the existing ones
  anyway, so it's safe to load from the file without
  overwriting the ones we already have in memory.
if/else -> guard clause
fix spec (don't use preview mode)+
if/else -> guard clause
extract content logic from font_parameters