Add admin setting for $CFG->latinexcelexport

Since MOODLE_16_STABLE we have one new setting to control if Excel files must use Unicode (UTF-16LE) or Latin (WIN-1252). This setting seems to fix a lot of reports about broken excel files, caused because the Unicode Excel format is, say, inestable.

So it would be great to add such setting in the admin interface, in order to allow people running under problems and being Latin1 to switch it and generate "stronger" excel files.

I leave this here because I don't know where to put it in and I'm pretty sure that your English will explain better the meaning of the setting.

TIA and ciao