Add CSS linting

[gsproston-scottlogic]

Feature Request

Description

Add linting rules for CSS styling/formatting. We can use Stylelint for this.

Additional context

From discussions in a PR, it would be nice to have some styling/formatting rules for our CSS files.
Stylelint contains two rules (here and here) which cover what is mentioned in the PR, but they are both included in the standard config, so just include that.

So, add the Stylelint standard config to the linting script, meaning CSS files should be linted when doing npm run lint on the frontend.
Once that's added, run the linter and fix any issues.

Acceptance criteria

Just formatting/styling changes, so regression testing here.