#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 
I wonder if we don't have a better place for this, looks like a generally useful function. @Raven24 ideas?

Also I think the term trim refers to removing whitespace in most languages that use it instead of strip. Let's be clear and name it truncate which is consistent with the Rails helper name.