[Imgur] Overhaul extractor module

[dirkf]

Boilerplate: own code, bug fix+improvement

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Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

• introduce base classes
• revise and refactor ImgurIE using posts/v1/media API for more accurate extraction
• add imgur: pseudo-URL-scheme
• revise and refactor ImgurIE using posts/v1/albums API, replacing defunct gallery API
• include valid tests from old PRs [Imgur] Fix issue 16577 #16674 and [imgur] support hidden albums and descriptions #21693.

Closes #25351.
Closes (replaces) #21693.