Sync My Unit per ammut

This Add-on uses the browser.storage.sync API to sync units.

At page load, it will compare local and synced state, and if out of sync, replaces local with remote state and reloads the page.

When the "Save Unit" button is clicked, the synced and local state is compared, and the remote state is replaced with the local state if necessary.

NO MERGING OF ANY KIND is being done, the "update strategy", so to say, is "replace".