R1:f5537fdff69f

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…