Replace jquery getJSON and get with native fetch

[absidue]

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Replace jquery getJSON and get with native fetch

Pull Request Type

• Feature Implementation

Related issue
Original giant PR #2360

Description
This PR replaces the calls to jquery's networking functions (getJSON and get) with the fetch API that is available in all browsers these days. It doesn't remove the jquery dependency as it's still used in quite a few other places in FreeTube, I'll change those in future PRs.

The idea is to split up my giant jquery PR into smaller more manageable ones, as that makes them easier to test and less likely that bugs sneak into them.

Testing (for code that is not small enough to be easily understandable)

1. Update check
   Set version in the package.json file to 0.16.0 and check that the "Update available..." banner shows up
2. Proxy test
   Click the proxy test button, if you have a proxy it will succeed. If you don't have a proxy you can check that the error handling works by looking in the devtools console
3. Switch to the Invidious API and just check that stuff still works
4. Enable SponsorBlock
   Video with segments: https://youtu.be/nFDBxBUfE74
   Video without segments: https://youtu.be/0bvZ70wgZVM
5. Enable captions on a video and see if they show up

Desktop (please complete the following information):

• OS: Windows
• OS Version: 10
• FreeTube version: 0.17.1