Phase out the 'management' permission

[astral-sa]

Since the old button extension is now part of the main extension, we aren't using the 'management' permission any longer. We should eventually remove it from the manifest.

The phase-out plan would be something along the lines of:

1. Release a version that uses the management permission one last time to remove the salr button extension, if installed.
2. Wait a while until people have auto-updated...
3. Remove the added code from (1) and the management permission.