[web] Make Input component box-sizing: border-box;

Summary: Noticed width of Input in LoginForm was behaving weirdly. Found this: https://stackoverflow.com/a/628912, and it seems like box-sizing: border-box is what we want for the Input component.

Test Plan: Input looks right in the context of LoginForm. I will look at other instances of Input in web to make sure that things continue to look as expected before landing.

Reviewers: def-au1t, palys-swm, varun, benschac, ashoat

Reviewed By: benschac, ashoat

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3502