Bug 28327: Unify CSV delimiter special behavior for tabulation

Bug 28327: Unify CSV delimiter special behavior for tabulation

System preference 'CSVdelimiter' has a special case for tabulation.
Preference value contains string 'tabulation' but string '\t' must be used in CSV file.

This is OK in many places, for exemple Bug 17590.

This patch adds C4::Context->csv_delimiter to add a uniq metod dealing
with this behavior.
Also create Koha::Template::Plugin::Koha->CSVDelimiter for calls from
Toolkit Templates.

Test plan :
1) Set system preference 'CSVdelimiter' = 'tabs'.
2) Create CSV export in impacted pages
3) Check columns are separated by tabulation character and not string 'tabulation'
4) Check with another delimiter

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>