Improved UX of signing pages

[klakhov]

Motivation and context

• Added input animation
• Removed additional prefix from inputs
• Fixed register form autofill
• Fixed some problems with autogenerated username
• Fixed text aligns on buttons, headings
• Decided to keep form button always active so user can press it, and see what fields have errors

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[klakhov]

/check

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[klakhov]

During running CI for this pr, ive encountered 2 unstable tests as reported here. They are not related, so we can merge I belive.