This PR removes most -- but not all -- of the remaining code pertaining to the now-removed tahoe magic-folder feature (which has since been superseded by the new standalone "Magic-Folder" application) and fixes/improves a number of small/miscellaneous things along the way, such as:

• Fixing/restoring the total "Stored:" indicator in the status panel to work with new Magic-Folder
• Fixing/restoring the "expected expiry" indicator (used on ZKAPAuthorizer-enabled grids) to work with new Magic-Folder
• Improving the "overall" status indicator to update immediately (instead of periodically, according to a timer) for new Magic-Folder
• Updating log-filters to conceal additional secrets (like author name, sign/verify keys) used by new Magic-Folder
• Fixing a potential crash condition caused by attempting to calculate file-sizes for files that were recently moved or removed from an active magic-folder
• Switching to new Magic-Folder's tahoe-objects endpoint for calculating ZKAP/token costs for (new) magic-folders
• Removing all sorts of hacks that were previously necessary to work around various (old) tahoe magic-folder bugs/limitations (and thereby hopefully restoring little bit of sanity for at least one developer...)