Issue 6833 - Add support for incremental filter list updates

• I wrote the patch so that diff ordering of - and + doesn't matter, but if we really want to force it to matter than I can make that change.
• I didn't optimize the part that integrates the diff in to the existing filter list, I figured we can do that later, but I can handle it now if desired.
• I decided that if a manual attempt follows a failed attempt that it should trigger a full list download, just because it seemed to make sense and was an easy change.
• diffs are required to use https or they are ignored.