implement sudo

[chadwhitacre]

Would be useful as admin to be able to use the site as another user, in order to handle support requests such as https://gratipay.freshdesk.com/helpdesk/tickets/1730. As with sudo, this should time-out after (say) five minutes.

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[Changaco]

Fixing the giving page (#3107 and #3219) would allow admins to modify a user's tips.