[Chore]: Stop tracking generated CSS in version control

[SleeplessOne1917]

Requirements

• [X] This is a feature request and not a bug report. Otherwise, please create a new bug report instead.
• [X] Please check to see if this request (or a similar one) already exists.
• [X] It's a single feature. Please don't request multiple features in one issue.

Describe the feature you'd like

I don't think we need to track auto generated CSS in the repo. I think all we care about are the scss files that are used to generate those css files.

[Nutomic]

This was previously attempted in https://github.com/LemmyNet/lemmy-ui/pull/2256 but then reverted in https://github.com/LemmyNet/lemmy-ui/pull/2259. So it needs configuration to auto-generate the files if they dont exist.