[web] cleanup compose subchannel modal

Summary:
This diff cleans up the one off styles in the compose subchannel modal and updates the modal to follow the conventions of the new modal api.

Linear task: https://linear.app/comm/issue/ENG-6735/add-member-list-for-subchannel-creation-is-not-reflecting-the-correct

Depends on D11065

Test Plan:
Please see the demo video below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11066