This patch is a first step to provide a preview mode for notice
templates.

CHECKIN, CHECKOUT and HOLD_SLIP are supported so far.
Maybe more, but I have not tested yet and the interface will not allow
you to generate the preview.

The idea is to provide an idea of how will render the messages generated
from a notice template.

A new "Preview" button is added close to each textarea on the editing
notice templates view.
For each notice template code (letter_code), we will need some input
data to produce the preview.

For instance, for CHECKIN we need an barcode. From the barcode we
can guess all the other data.
For CHECKOUT we will need the borrowernumber and the barcode.
Note that the way to enter the data for the preview is not really
user-friendly, for CHECKOUT you will have to fill
'borrowernumber|barcode', but the placeholder will help you to know how
and what to fill.
In the modal window, you will see 4 blocks:
1/ the content of the letter (with the placeholds << >>)
2/ the generated message (with the data filled)
3/ if the letter contained historical syntax markers, the screen will
try to generate a notice template using the TT syntax
4/ the generated message from this TT syntax
=> You will be able to compare the 2 generated messages.

What is the goal of this first patchset:
- Show this first POC and get feedback from other developpers
- Add a way to easily visualise the differences between the 2 syntaxes
- Confort users with the TT syntax and the migration step from the
historical syntax.

I'd like to get opinions before going further.

The possibilities:
- Mock data to get fully working generated messages for any notice
templates. For instance, for CHECKIN and CHECKOUT, the item is not
checked in/out yet. So we cannot access the issue's information.
(I have no idea how to do that)
- Browse the data to get the ones we want to use for the preview (big).

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>