Add a 'description' field to Phriction

Summary:
Add a new column to PhrictionContent called 'comment' or 'description' or
something

Add an optional field to the Phriction document editing interface that

allows you to add a comment

Test Plan:
Run the sql patch to modify phriction_content table

Create a new wiki page in Phriction
Type in words in description field and save the page
Visit this page and click "Edit Page" button
The content in the desciption field is saved

Reviewed By: epriestley
Reviewers: epriestley, hsb, codeblock
Commenters: codeblock
CC: aran, codeblock, hwang, epriestley
Differential Revision: 709