Merge branch '200-duplicate-entries-in-contactlist' into 'master'

Fix contacts duplicating itself when pressing back button in conversation view.

Normally, this should be handled by the `SortedList` in the `RecyclerView`, but it isn't for some reason. Instead of spending way too much time on finding out why, I am clearing the adapter instead (on each `activity.onResume()`) as it was done before.

Closes #200

See merge request !42