Bug 17960: Rename opac_news.new with opac_news.content

Bug 17960: Rename opac_news.new with opac_news.content

The field opac_news.new is very confusing and should be renamed.
If you want to access it via Koha::NewsItem you will have trouble:

  use Koha::News;
  my $news_item = Koha::News->next;
  say $news_item->new;

=> Attempt to bless into a reference at /home/vagrant/kohaclone/Koha/Object.pm line 78.

This patchset is going to rename this DB field to opac_news_content instead.

Since the opac_news.new can be used in notice templates, we need to warn the
user during the update DB process that some templates must be updated.

Test plan:
0/ Apply the first patch "Add a test to highlight the issue" and confirm that
the test fail
1/ Apply this second patch
2/ Execute the DB entry
3/ Confirm that you get a warning if at least one of your notice templates is
using opac_news.new
4/ Confirm that the test new pass
5/ Add/update and delete a news
6/ Confirm that the RSS new feed still works as expected

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>