[web] Rename NotificationsOption -> EnumSettingsOption

Summary:
We want to reuse the (previously known as) NotificationsOption component for the "Privacy" tab of ThreadSettingsModal.

As a first step, we rename NotificationsOption to more generic EnumSettingsOption (open to alternative names). In later diffs will remove "Notifications-specific" parts of EnumSettingsOption and make it more generic.

Made sure to rename component, rename file, and fix imports.

Depends on D4044

Test Plan: Opened NotificationsModal and it continues to look/work as expected. Also flow/eslint.

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4045