[web] Description box shouldn't be resizable horizontally

Summary:
Set the textarea to be resizable only vertically through it's supported CSS properties. Also set a min-width so it isn't resizable small
enough to cut out the description.

Linear: https://linear.app/comm/issue/ENG-2016/description-box-shouldnt-be-resizable-horizontally

Test Plan:
Works as expected, and tested before & after this change.

Before

After

Reviewers: atul, ginsu, michal

Reviewed By: atul

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5388