[web] Introduce additional Menu styling variants

Summary:
Add additional styles for Menu, so it could be used in modal (where the menu design is a bit different)
Default styling (in thread actions menu) remains the same, although the menu will a bit lower than it should. After the next diff (moving rendering into portal) it will look ok.

Menu with thread actions:

Menu in modal:

Test Plan: Newly added styles are not used yet, but current thread actions menu should remain the same (only its position is not exactly correct)

Reviewers: palys-swm, benschac, atul

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, atul, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3376