Download diff handler

[DubyaDude]

Currently when downloading assets, the downloader will always download even if it's already downloaded the same thing.
This has caused some users to 'accidentally' cause excessive downloads to the point that the administrators have contacted them.

The booth downloader should handle the following:

• See if there are any changes in the booth downloadable links, which should trigger a full redownload (new versions/purchases)
• See if any changes in images occur, in which case both images and booth json should be updated.
• See if any changes in the booth json occur, in which case only the json will be updated.
• If no changes occur, do a skip
• A parameter to force full redownload, however, we should generally advise against using this for mass DL.

This can be achieved by keeping a cache of the booth items downloaded, see BoothBatchDownloader _Debug_BatchDownloadDebug.json

FOR THE TIME BEING

Since maintainers are currently busy with some other things, advise users to only download/redownload what they need.