Implement activity stream and messages

[pjotrsavitski]

Activity stream should be displaying all activities for the episode.

• Panel should have a switch that would allow showing either messages or activities.
• Up to 50 newest items should be loaded for both messages and activities (additional items would be loaded when scrolling to the end)
• New activities and messages should be added to the top (using data transmitted by WS connection)
• Both activities and messages should be loaded when episode view is opened
• It should be possible to click on activity item and move the timeline to that item (the time representation of that item, if it still exists). It might be useful to check if that item still exists or not.

[pjotrsavitski]

Only last point is not implemented, although it already has a standalone issues.