Gracefully handle non-presence of Place Hold link

[Jeff Davis]

The _replace_place_hold_link function in od_pages_opac.coffee assumes the existence of a Place Hold link:

$a = @find '.place_hold > a'

But in a stock EG install, e-resource titles do not have a Place Hold link, so you get an error "TypeError: $a.prop(...) is undefined".

At minimum, that function ought to check for the existence of the element before trying to do anything with it. (We do probably want a Place Hold/Check Out link on Overdrive records even if there is no underlying Place Hold link to build on.)