[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