Improve Conversation Scrolling Behaviour

Currently, we scroll down after loading the list of messages (after each activity start). We also scroll down when loading a message text or image asynchronously.

This is problematic, because the user can't scroll up right after a large conversation is displayed and because when looking at older photos, the user forcibly gets scrolled to the end of the conversation and needs to find where she was.

Subtask of #1242 (closed)