Review Mozilla 1947828: Populate the "Add signature" menu with the stored signatures

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1947828

If we are in private browsing mode then we should not offer to save signatures. This feature is accessible via the built-in PDF viewer and is present in Tor Browser Nightly.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information