Multiple comment submissions for a diff is prevented now.

Summary:
It was possible to submit a comment multiple times if the submit
button was pressed more than once quickly. Added javascript code
that disables the button when it is clicked.

Test Plan:
Tried to click the button multiple times very quickly, but the
button was disabled after the first click.

Reviewed By: epriestley
Reviewers: epriestley, jungejason
Commenters: aran
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 337