[web] Block sending multimedia messages to pending threads

Test Plan:
Open pending thread. Check if multimedia input is not displayed. Drag and drop an image file and check if it wasn't added.
Create real thread and test again. Now it should be possible to add an image using input and drag and drop.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D986