Bug 25776: Add a 'last updated' column for notices

Bug 25776: Add a 'last updated' column for notices

This add an 'udated_on' column to the letter table and displays
the information in 'slips and notices'.

To test:
- Apply patch
- Run updatedatabase
  Note: the existing letters will all be set to the
  currnet date).
- Go to Tools > Notices and slips
- Verify there is a new column "Last updated" in the table
  Note: The table will always display the newest change date
  of the different message transport types.
- Edit any notice
- Verify the "Last updated" is shown for any transport type
  with an existing notice definition
- Edit and add a notice
- Verify the dates are correctly stored
- Test the column configuration for the new column works
  correctly

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>