Add default presentation

[cheuberg]

Polls can only be used when there is a presentation, therefore I think that having the default presentation available can be useful.

Note that the current nginx configuration in this repository does not specify where http://DOMAIN/default.pdf actually resides, so nginx's default root html is used.

[alangecker]

@cheuberg thanks for the pull request! :)

• with an an old .env file the DEFAULT_PRESENTATION variable will be missing after upgrade leading the nginx complaining/crashing. I've added a default value ${DEFAULT_PRESENTATION:-/dev/null}, which makes it also possible to unset the variable if you don't want any presentation by default.
• If someone would replace the pdf in ./conf, it would lead to git merging issues. I moved the file now to ./mod/nginx, since the ./conf folder was intended to be for configurable files only and with an entry in .gitignore it is now possible for the admin to place pdf's in there without git issues.
• Currently already a lot of people rely on the stability of v2.2.x. I've changed the base to the new branch develop, which gets deployed to my personal instance for testing purposes. As soon as it stays stable for a few days I intend to merge it to v2.2.x