Add back comma after last name in item edit pane

[dstillman]

(In [661]) Closes #247, Add interface option for institutional creators in item edit pane

Here's a shot at a single/double creator field toggle switch -- let me know what you think

A few issues:

• There's currently no comma between the last name and first name when in two-field mode -- I removed it to greatly simplify the code, hoping to be able to use the CSS :after pseudo-element, but that seems to not work with XUL -- I'll figure out a clean solution and add it back ( refs #288 )

• It's not very smart about switching between single-field mode and two-field mode, as it currently just keeps the last word (even if it's "Jr." or "III") as the last name and puts the rest in the first name field -- not a big deal, but it should at least be a bit smarter about this ( refs #289 )

• There are probably some other bugs

[dstillman]

(In [692]) Fixes #288, Add back comma after last name in item edit pane