Add related contact to offices index

Displays related_contact &/or general inquiries contact info on offices index page.

- Moves contact-us code into macro for reuse here. (Macro currently still lives in contact-us directory)
- Uses single query to fetch both related_contact & 'general_inquiries'. May need to change if there is a preference in display order.
If we're going to use this macro in areas other than `contact-us` should we just put it in the `_includes` or `_layouts` folder with the other macros?