Crop screenshot thumbs in Maniphest

Summary:
If you name an attached file something like
"derpderpderpderpderdperdperpereprederpderplolololololo.png", the display
overflows. Crop and shorten it so that it looks decent. Show the full name as
the image title.

Test Plan:
Named a file "derpderpderpderpderdperdperpereprederpderplolololololo.png" and
got a sensible display out of it.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, tuomaspelkonen
Differential Revision: 389