Feedback improvements: API throttling issues, support deleting multiple images, and more

[daveschumaker]

• (fix) Issue with incorrect or partial image counts being downloaded from a completed pending image request.
• (fix) Issue with "2 per 1 second" rate limit when requesting new images
• (feat) support AI Horde's "allow_downgrade" which will attempt to downgrade an image request if parameters end up requiring too many kudos to complete (useful for anonymous users)
• (feat) re-enable support for multi-select images in the image gallery, allowing you to delete multiple images at once.
• (fix) image gallery would sometimes randomly appear to duplicate images due to incorrect use of job id's as a unique key (one job can have multiple images associated with it).
• (fix) No longer cancels pending job if AI Horde replies with "is_possible: false" -- it will keep trying (in hopes that a machine comes online to support the request)
• (feat) when downloading multiple images from the new select tool in the image gallery, ArtBot creates a helpful html file that allows you to view the locally saved images offline in a web browser.
• (fix) jobs with "is_possible" equal to false are in a stuck state and don't seem to request updates from server.
• (feat) add new accordion option to section component and re-arrange image upload options on advanced settings page
• (fix) issue ensuring we cast certain parameters to valid type expected by Horde API (e.g., sending 2 instead of "2")
• (fix) more work handling stuck jobs when "is_possible" is false. It really works, this time!
• (feat) add ability to cancel job in progress without losing existing downloaded images (useful for jobs where GPU worker goes offline).
• (feat) likewise, if a job returns a 404 error after taking too long to complete, you will no longer lose existing images related to the job if they had already been downloaded.