[web] Link modal in the thread menu

Summary:
The thread menu should allow the user to open up the media gallery.

Linear: https://linear.app/comm/issue/ENG-2879/link-modal-in-the-thread-menu

Depends on D6863

Test Plan:
Confirmed that the menu appears and pushes the modal on screen

Reviewers: atul, ginsu, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6864