Add configuration to disable lending buttons

[jimchamp]

Adds disable_lending configuration and new public function that returns this configuration's value.

Adds new UnavailbleButton macro, which is returned by LoanStatus when disable_lending is true.

The suggested call to action overflowed for all buttons in desktop views. As a result, the copy has been updated to read "Currently Unavailable." Unfortunately, this copy overflows in carousels, but fits in other contexts.

Book page desktop

In carousel

Technical

Testing

1. Set the disable_lending configuration to True
2. Verify that lending buttons have the expected call to action.

Screenshot

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