LP 1772993: Ability to blank Loan Duration in Vol/Copy Editor.

LP 1772993: Ability to blank Loan Duration in Vol/Copy Editor.

Add the <NONE> option to the loan duration field in the web staff
client Vol/Copy Editor.

NB: This was split into its own commit in case it is decided that this
change is not wanted.

To test:

1. Add a value for loan duration to a copy.

2. Attempt to remove the value in the copy editor.  Note that the
   dropdown does not allow you to do so.

3. Apply this commit.

4. Repeat step 2.  Now there is a <NONE> option that removes the value
   from the copy.

Signed-off-by: Jason Stephenson <jason@sigio.com>