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### Motivation and context
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As far as I can see, this is the only request that the UI makes in the
`application/x-www-form-urlencoded` format, and there's no reason for it
to do that. Switching to JSON makes the code more readable and
consistent with other code, and enables us to retire
`application/x-www-form-urlencoded` support in the server, if we decide
to do so.

### How has this been tested?
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I manually checked that logging in still works.

### Checklist
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- [x] I submit my changes into the `develop` branch
- ~~[ ] I have added a description of my changes into the
[CHANGELOG](https://github.com/opencv/cvat/blob/develop/CHANGELOG.md)
file~~
- ~~[ ] I have updated the documentation accordingly~~
- ~~[ ] I have added tests to cover my changes~~
- ~~[ ] I have linked related issues (see [GitHub docs](

https://help.github.com/en/github/managing-your-work-on-github/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword))~~
- ~~[ ] I have increased versions of npm packages if it is necessary

([cvat-canvas](https://github.com/opencv/cvat/tree/develop/cvat-canvas#versioning),

[cvat-core](https://github.com/opencv/cvat/tree/develop/cvat-core#versioning),

[cvat-data](https://github.com/opencv/cvat/tree/develop/cvat-data#versioning)
and

[cvat-ui](https://github.com/opencv/cvat/tree/develop/cvat-ui#versioning))~~

### License

- [x] I submit _my code changes_ under the same [MIT License](
https://github.com/opencv/cvat/blob/develop/LICENSE) that covers the
project.
  Feel free to contact the maintainers if that's a concern.

Co-authored-by: Nikita Manovich <nikita@cvat.ai>