[web] Change Button component color on hover

Summary:
I looked through Figma and couldn't find any buttons in the "hover" state... but I still think buttons should change color on hover and it feels unnatural when they don't.

The button background was --violet-dark-100, so I went with --violet-dark-80 for background color on hover for now

Test Plan:
Looks as expected:

Reviewers: palys-swm, def-au1t, benschac

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3557