Always show inline comment column in Pholio

Summary:

• Always show the column so there's no jumping around when comments are added.
• Set width to 320px so we can mobile it (not 100% sure how this will work yet).
• When there are too many comments, scroll them internally.

Test Plan: {F33552}

Reviewers: ljalonen, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5071