Revert "[web] Block sending multimedia messages to pending threads"
Summary: This reverts commit 52fb8a4d and diff D986
Test Plan: Check if it's possible to send multimedia to pending threads (using inout and drag&drop)
Reviewers: ashoat
Reviewed By: ashoat
Subscribers: KatPo, Adrian, atul
Differential Revision: https://phabricator.ashoat.com/D1101