Fix for Bug 4504, Confirmation messages in opac account not translated

Fix for Bug 4504, Confirmation messages in opac account not translated

Strings within JavaScript onclick attributes are not getting picked
up by the translator:

onclick="return confirmDelete(_('Are you sure you want to cancel this
hold?'));"

This patch converts those to use variables defined in the main
script block:

onclick="return confirmDelete(MSG_CONFIRM_DELETE_HOLD);"

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>