Add activity view for displaying user specific data [feature/activity-view]

:exclamation: This branch is under development and shall only be merged with caution :exclamation:

The intention of this pull request is keep track and discuss an activity view as implied by the tabbar.
The view will in this current vision consist of a simple UITableView which renders either OPFQuestions, OPFAnswers and OPFComments in a simple, non space consuming manner. A click on any cell will lead to its corresponding Question/Answer- and Comment-View.
Data displayed will or course be always user bound wherefore the app needs state about a logged in user.

Groet'n, Tobi