[Frontend] Cleanup CSS

[shu8]

Describe the bug The frontend refactor (https://github.com/uclapi/uclapi/pull/4114, https://github.com/uclapi/uclapi/pull/4095) migrated most of the original CSS, but a lot of it may be duplicated or redundant now after the refactoring.

It would be nice to go through the CSS and clean up all the unnecessary styles.

It would be even better to convert most of the styles to use CSS modules (see Next.js docs) rather than having most of the styles as global styles. Some of the components already use this, but a lot of styles are still global.