[web] Introduce plain Button variant

Summary:
Part of ENG-843
Introduces plain Button variant with minimal css properties. It becomes the default variant, so this diff also changes any Buttons without explicit variant to use filled.

Also makes the onClick nullable and sets the type to button to make it more general.

This variant is used in a later diff.

Test Plan:

• Run flow
• Check if all changed buttons look and work the same
• Check if there is any <form> that uses a Button without an explicit type set

Reviewers: tomek, atul, abosh, ginsu

Reviewed By: tomek, atul, ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5342