Allow and style file block

[snim2]

Corse Parish Council use this theme and have a page which lists their minutes:

https://www.corse-pc.gov.uk/parish-council/minutes/

There are two issues with this:

1. The "file" block has been removed from this theme so every update requires a manual change to the theme code
2. The file block is not styled per the design system and as a result is not accessible and also not consistent with the rest of the site

This PR fixes these two issues, and also styles the core "buttons" block which had a similar issue.

Design system reference: https://design-system.service.gov.uk/components/button/

Testing

1. Spin up the local env with this theme on this branch (saluki-test-site has this theme installed)
2. You may need to add this to your functions.php if you want to upload a PDF:

   function allow_pdf_uploads($mimes) {
       $mimes['pdf'] = 'application/pdf';
       return $mimes;
   }
   add_filter('upload_mimes', 'allow_pdf_uploads');

3. Visit http://localhost/wp-login.php and log in
4. Visit http://localhost/wp-admin/edit.php and add a new post
5. Add a button block
6. Add a file block
7. Check that they are (roughly) styled as per the system in the block editor
8. Check the page in the front end and check the styling is correct

Screenshots

Before

Editor:

Front-end:

After

Editor:

Front-end:

[matthewpassmore]

Looks good locally 👍