[web] Change how margins / paddings are used in message result

Summary:
I need to change how margins / paddings are used in message result, because I will be using this component with our SearchModal component that has some default margin. So I move the responsibility of setting the margins / paddings to the outer container.

Additionally the margin in TogglePinModal gets fixed: it seems to me from the designs that there should be a space between the message and the top text. @rohan please tell me if I'm misunderstanding

Test Plan:
Before:

After:

Reviewers: rohan

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8207