[web] Remove unnecessary form_footer div and corresponding styles

Summary: There's no need for this div. We can achieve the same thing with just the modal_form_error div.

Test Plan:
Looks as expected:

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3926