bug 1433: use same bib when adding new subscription from details page

bug 1433: use same bib when adding new subscription from details page

When viewing the subscription details page of a bib that already
has at least one subscription, clicking on the 'New Subscription'
button in the toolbar will fill in the biblionumber and title
in the new subscription form.

Clicking on the 'New Subscription' button in other context (e.g.,
when viewing the results of a search) will leave the biblionumber
and title fields unpopulated, as there is no specific bib to
use.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>