Detect internal users and add an extra section to settings for them.

[sammacbeth]

Reviewer:

Description:

This mirrors how we detect internal users on other platforms (for example apple platforms, in order to add a isInternalUser flag to settings. This can be used to expose additional settings in the options menu.

Steps to test this PR:

1. Open settings - note that the sections are as before.
2. Visit use-login.duckduckgo.com and login with DDG SSO.
3. Return to the settings page - there should not be a new 'Interal settings' section.

[kzar]

Changes LGTM, but as mentioned in Asana I want to double check about a privacy review before we merge this. Edit: Privacy concerns addressed in Asana.