Removes end voting button from planning poker button panel
change button labels based on session status
Adds functionality to open/end voting/close session buttons on overview panel
Resolves errors resulting from rebase
Only enables buttons for a session if it is being displayed

This is a re-application of an earlier fix that seems to have been overwritten in this branch.
Merge pull request #273 from TheTeam8s/addButtonsToOverview

Add buttons to overview