Bug 11848: Fix C4::Context::interface, add POD and UT

Bug 11848: Fix C4::Context::interface, add POD and UT

1/ Edit a Perl script, for example mainpage.pl
2/ add "use Koha::I18N;" to the top of file
3/ add a translatable message somewhere in the script (this have
   to be after the call to get_template_and_user). For example:
   warn gettext("This is a translated warning");
4/ Create or update the PO files with
   misc/translator/translate create LANGCODE
or
   misc/translator/translate update LANGCODE
   (LANGCODE should be enable in syspref 'languages')
5/ In misc/translator/po/LANGCODE-messages.po you should have
   your string, translate it (using a text editor or a PO file
   editor, make sure you don't have the "fuzzy" flag for this
   string).
6/ Go to mainpage.pl with active language being English with your
   browser and check your logs. You should see your string "This
   is a translated warning".
7/ Now change language to LANGCODE. Check your logs, you should
   have the string translated.

Note: I chose to name the sub 'gettext' because it's the default
keyword for xgettext for Perl. We can change it to whatever we want.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Follow test plan, work as described.
No koha-qa errors.
Tests pass

Fixed small merge conflict on t/Context.t

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests and QA script.
Copied test plan from bug.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>