Make the drop target for file attachments smaller

Summary:
Currently, in Maniphest, if you drag-and-drop a file it always attaches. Instead, I want you to have two options:

• Drag and drop to the attachment area to attach; or
• drag and drop to the Remarkup panel to upload + inline.

For the first step, make the input have a clear drop target instead of it being the entire panel.

Test Plan: Attached files in Create Task, task view, meta mta send test.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3651