#4930 added trim of opengraph description to 250

Trimmed opengraph description in mobile view and stream view to 250 chars. SinglePostView still has the long description.

URL used to test (but I also added a jasmine test): http://www.kotzendes-einhorn.de/blog/2014-10/krautstart-die-stimmen-zum-launch-von-krautreporter/

Resolving bug #4930 
we could put it directly on `app.helpers` as `app.helpers.truncate`. that might even justify putting it in its own file.