convert user wiki pages of the user meetings

I have converted the old wiki entry of the 2015 Dune User Meeting to Markdown and corrected the links in meetings.md and 2015-09-devmeeting.md.

What will happen to the slides of all the talks? They currently reside under http://users.dune-project.org/attachments/download/. Will this be deleted with the user wiki, or can those files remain there?