Do not use "Full name" in exported quiz results sheet

When exporting results of the quiz, the first column of the sheet is labeled as "full name" and it contains the full name of a student. In Moodle, the string "Full name" is primarily used in the context of the course title. In some languages (including Czech), you can not use the same words for "Full name" (the course title) and "Full name" (the name of a person).

Therefore:

• either create new $string['fullpersonname'], probably in the global context (moodle.php)
• or split the column into two - First name and Last name (Surname), which are primarily used in the context of somebody's name.