Strip all trailing slashes, to ensure folders are stored correctly

This has been an issue for a while, but I totally forgot about it.  If you specify media folders with a trailing slash like so:

`"mediaFolders": ["/srv/media/Music/FLAC/Atreyu", "/srv/media/Music/FLAC/Boston/"],`

The "Boston" folder will not appear in `/api/folders`, but its contents will appear in `/api/albums`.  This fixes that.