[web] Redesign Modal to match Figma

Summary:
Redesigned modal to match existing designs. Refactored and simplified CSS classes and removed redundant container in component.
Removed fixedHeight property, as it was used in one place and had no effect on existing implementation.
Existing modal sizes (small, large) should work as before. Introduced fit-content size option, to allow adjust modal size by its content.

Test Plan: Opened currently existing modals. It's size should remain unchanges, but fonts and design should now match Figma.

Reviewers: palys-swm, atul, benschac

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3669