Allow patrons to "Request a Book" from libraries

[datalogics-deh]

Implement a method that allows patrons to request books from libraries.

For example, an additional item under Settings > Library (alongside Content License, Privacy Policy, and Report an Issue), which would allow the user to send an email to the library (option to use a different email address than Report an Issue).

[leonardr]

Server side component of https://github.com/NYPL-Simplified/Simplified/issues/68. The simplest thing to do is to add an email address to the library configuration which shows up in the Authentication For OPDS document.