Cancel downloads in progress when threads removed

[Issung]

New download managers from bugfix/imagelink-download-duplicates-raceconditions-threadsafety branch will allow cancellation of downloads in progress, and stopping future downloads too.

A few things are needed:

• Need to use a download technique that supports cancellation. Probably need to swap to using HttpClient.
  • Will also need to adjust all download exception handling.
• When user changes "SaveHTML" setting, cancel thread html downloads if false.
• When a thread is removed, cancel image downloads and cancel html download.
• Cancel when user initiates removal, so no more exceptions happen regarding renaming folders.

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