CSV export: add UTF-8 BOM header.

When exporting a poll to CSV, no BOM character is written. Most applications can
properly detect the required encoding, but some do not, and might find it
helpful to be able to read a BOM character.

--
Comment added 2019-04-16, nico@bou.io:

The issue is specifically with Excel that assumes Latin-1 by default, but switches to utf-8 if a BOM is present. On the other hand, LibreOffice and other apps seem to behave correctly with a BOM.

exportcsv.php

@@ -73,6 +73,7 @@ $slots = $pollService->allSlotsByPoll($poll);
 $votes = $pollService->allVotesByPollId($poll_id);
 
 // CSV header
+echo "\xEF\xBB\xBF"; // BOM character for UTF-8
 if ($poll->format === 'D') {
     $titles_line = ',';
     $moments_line = ',';