Create Dukecon client supporting multiple conferences

[michalharakal]

Create a app variant supporting all dukecon.org conferences. Now there als specific app for every particular conference. Dukecon API already provide endpoint for supporting multiple conferences from one service. To achieve that:

1. add proper support for one or multiple conference sessions at once
2. Implement rule for providing "default" conference
3. Implement UI for selecting a conference avaliable from API
4. Optional After backend provides API for "default" conference, implement this as a separate issue.