Bug 5456 - Create a link to opac-ics.pl

Bug 5456 - Create a link to opac-ics.pl

opac-ics.pl was added to Koha back in 2007, but there has not been a link to it
from anywhere. This patch adds the link from the main view on
/cgi-bin/koha/opac-user.pl, and makes some changes to opac-ics.pl itself.

Changes to opac-ics.pl:
- Update the code to use date and time for when a loan is due
- Use dt_from_string to turn a MySQL time and date into a DateTime
- Add a timestamp and a UID (these were reported as necessary by a couple of
  iCal validators)
- Remove DTEND from events, since we now have time as well as date for when a
  loan is due
- For loans that are overdue, set the DTSTART to now(), to avoid creating
  calendar events in the past
- Use a template to generate text strings, to make them translatable

To test:
- Apply the patch
- Make sure you have at least a couple of loans, including one overdue
- Go to /cgi-bin/koha/opac-user.pl and click on the link labeled "Download as
  iCal/.ics file"
- Inspect the file in a text editor and/or view the result in a calendar
  appplication. Make sure the data makes sense.
- Sign off and go thee merrily on thine way.

Update 2016-07-25
- Force date_due to UTC to avoid problematic timezone info like this:
  DTSTART:TZID=Europe/Zurich:20160724T235900
- Remove quotes from summary and description

Followed test plan, koha.ics file successfully imported to Thunderbird calendar
Signed-off-by: Marc <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>