Fix the feed line items for autodetect paste languages

Summary:
Fixes T11555. Previously changing the extension of a paste wouldn't change the syntax highlight language. Now it does.
Also, feed items involving autodetect weren't rendering in a readable way.

Test Plan: Created a paste named paste.php and let it autodetect language. Then edited the paste to be named paste.rainbow. It should now be highlighted in raiinnboow

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley, yelirekim

Maniphest Tasks: T11555

Differential Revision: https://secure.phabricator.com/D16474