Remove Imgur support

This issue is to track the removal of Imgur support, as of now it's only deprecated as a replacement custom action is starting to be shipped. After a couple releases I plan to remove all code to handle Imgur uploads, only the custom action will remain for some time, but ultimately that will not be shipped either. After that users will have to add their own custom actions or packagers can include what they deem best for their distribution.

Please provide feedback here, just keep in mind that as an open source project we're in an awkward position to support a proprietary service out of so many out there. Add to that the recent controversial changes in their Terms of Service. In the past ZimageZ was supported but that service just disappeared, leaving screenshooter with a broken feature for some time.

I expect many people are going to ask to add support to certain hosting services, e.g. "At least add support to S3, it's the industry standard" or "You gotta support IPFS, it's open source, distributed, no way that's going to vanish anytime soon", my stance is that this is a slippery slope, if one or two services have built-in support, then I'll have a hard time rejecting further requests, in the end that would be unfair or I'd have to maintain more code to upload screenshots than taking screenshots.

With custom actions users have the power to do whatever they need with the screenshots in a flexible and convenient fashion, I'll be more than happy to add clever examples to the documentation upon requests.