Bug 24662: Remove global variables MSG_* from datatables.inc

Bug 24662: Remove global variables MSG_* from datatables.inc

Now that bug 21156 is pushed, we don't need to have global variables in
.inc or .tt files for translation in .js file. We can simply declare our
translatable strings where they are used.

This patch removes all global variables used in DataTables configuration

Test plan:
1. cd misc/translator && ./translate update fr-FR
2. Translate strings in misc/tranlator/po/fr-FR-messages-js.po
3. cd misc/translator && ./translate install fr-FR
4. Go to the staff interface, in english, and check that DataTables
tables are still working. You should check at least the following pages:
    - catalogue/detail.pl
    - circ/circulation.pl
    - tools/quotes.pl
    - tools/letter.pl
5. Switch to french and check again DataTables tables, and verify that
   strings are translated

This patch depends on bug 24661

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>