Implement DMActivityHandler

Summary: This differential implements DMActivityHandler component. Its role is the same as ActivityHandler but it works thick threads. I opted for new compoment instead of modifying ActivityHandler since we only need one component for thick threads while there are many ActivityHandlers (one for each keyserver). Therefore I couldn't see an easy way to reuse ActivityHandler.

Test Plan:

1. Create thick thread between users A and B.
2. Log in as user A on two devices (on web and on native) and log in as user B on third device.
3. Send message from B to A. Open the thread as A on web. Ensure rescinding happens on native.
4. Open the thread as A on native. Change opened thread as A on web. Send messages from B to A. Ensure that thread remains read on web.

Reviewers: tomek, kamil, inka

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13246