Improve Conpherence column textarea JS behaviors

Summary:

• Don't show a loading state on the whole column while sending chat. We could show some kind of minor loading state, but standard JX.Busy stuff will kick in after a couple seconds anyway.
• Blank the textarea immediately on submit so you can start typing more text.
• Don't disable the form while submiting; disabling it prevents you from typing more text.
• Hide the placeholder while the textarea is focused. If we don't do this, the placeholder reappearing after submitting text feels weird to me.

Test Plan:

• Sent a lot of text.
• Real fast.
• Focused and unfocused the area.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12086