Remove useStyles, refactor into StyledComponents and sx

[Tymek]

useStyles/makeStyles() is deprecated (See: https://mui.com/system/styles/basics/).

We need to migrate to styled, or where it makes sense use sx while keeping in mind performance tradeoffs.

Examples

New syntax: https://github.com/Unleash/unleash/blob/228ddabf73411ce8837824de1e15fe58659896a8/frontend/src/component/addons/AddonForm/AddonForm.styles.tsx

Old syntax: https://github.com/Unleash/unleash/blob/228ddabf73411ce8837824de1e15fe58659896a8/frontend/src/component/admin/apiToken/ApiTokenForm/ApiTokenForm.styles.ts

[asharonbaltazar]

Hey, @Tymek, can I take a shot at this? Seems like a good first issue to orient myself with the codebase.

[Tymek]

This can be worked on by multiple people. Pick couple of files, let us know in the comment, and go for it! :) Once small initial contribution is reviewed you can choose to do another batch, that's how I see it.

[ShaswatPrabhat]

Picking up: frontend/src/component/admin/users/UserForm/UserForm.styles.ts

Will share PR shortly

[ShaswatPrabhat]

Please find the PR UserForm.tsx

[stale]

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