[web] Introduce enum-settings-option[-info].css stylesheets

Summary:
Separating the styles for the NotificationsModal, EnumSettingsOption, and EnumSettingsOptionInfo components.

This diff works towards decoupling the EnumSettingsOption component from NotificationsModal... specifically so it can be re-used for the ThreadSettingsPrivacyTab component.

Depends on D4045

Test Plan:
Close reading, opened NotificationsModal and it seemed to look/behave as expected:

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4046