Include user's index preference in account API

3rd party sites allow indexing of user profiles. Perhaps they will
implement noindex if the user's preference is easily accessible. This is
an implementation of this request:
https://www.reddit.com/r/ideasfortheadmins/comments/2ktryi/allow_reddit_clients_to_respect_users_noindex/
Set HTTP header X-Robots-Tag to noindex

For the account API, if the user prefers not to be indexed, set the HTTP
response header `X-Robots-Tag` to noindex. If the user has no preference
on indexing, then the header is never set.