Change repository description from plain text to remarkup

Summary:
At one point this was sort of a one-line summary but it isn't really anymore (and doesn't appear on the list view). We could add a summary in the future if we wanted.

• Change the control from a text area to a remarkup area.
• Change the display to remarkup.

Test Plan:
{F44183}
{F44184}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6034