[web] introduce keyservers to the settings switcher

Summary:
This diff introduces a new "Keyservers" option in the settings switcher. This option will be gated behind the staffCanSee flag. Subsequent diffs will implement the keyserver list ui

Here is a screenshot of the figma designs:

Linear task: https://linear.app/comm/issue/ENG-4720/create-new-keyserver-selection-setting-panel-in-the-user-settings

Test Plan:
Please see screenshot below

Reviewers: inka, rohan, michal

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9887