custom css from settings app, switch to secure subdomain

[dr-frmr]

Problem

Users should be able to customize the look of their OS.

Solution

This PR adds the ability to customize kinode.css, a shared stylesheet relied on by all apps served in the base distro, via the Settings app. Edits to the stylesheet are persisted on disk. Users can go crazy with their own custom styles now.

Testing

1. Boot node
2. Navigate to Settings, edit the stylesheet (at bottom of page).
3. Refresh page, nagivate to other apps/pages
4. (optional) reboot node to see persistence

Docs Update

N/A

Notes

I've moved the settings app to a Secure Subdomain. It always should have been on one, but the tech is stable enough now. This prevents other apps from using its HTTP endpoints to manipulate the stylesheet without permission (and other much worse actions!)