There was duplication and a ugly `#if os(macOS)` with some footers
explicitly rendered on macOS with .themeFooter() (now .themeSubtitle()).
Move this logic to the .themeSection() modifiers by adding a
`forcesFooter` parameter. When true, a fake footer is added on macOS as
the last row of a section.
Some providers require specific credentials for OpenVPN, different from
account credentials. Update the API index with this information to show
an information footer and possibly a link to the OpenVPN credentials.
Also, fix the OTP footer not appearing on macOS.
Davide