The last confirmation step during poll creation flow is identical
between date polls and classical polls.
This commit moves the code in the `PollService` to be used in both
creation poll controllers.
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.
