County file download buttons

[ranweiler]

Users are now able to download ballot manifest and CVR export files for (1) their own county (if a county user) or (2) all counties (if they are a DOS user).

• The download buttons for the county dashboard is on the county home page.
• The download buttons for each county for the DOS user is in the county detail page, e.g. /sos/county/1 for Adams county.

The UI for this was implemented in #771, and now needs to be documented.

[kiniry]

Ping @mkiniry.