Tidy up CSS and move out of template

[cjrace]

We currently have one long custom CSS file to style the dashboard. Though we should try to clean this up / get rid of what isn't needed anymore:

Remove all unnecessary CSS from www/dfe_shiny_gov_style.css without altering the appearance of the app by...

1. Make use of what is already attached in shinyGovstyle from the GDS design system, using functions like gov_text() to pull it in instead of relying on a CSS file in the repo

2. Move any CSS for specific components that remains into the dfeshiny package, attached to those specific components

Remove as many instances of !important as we can

• !important is generally bad practice, and can usually be avoided by using the appropriate specificity for the CSS assignments