ConversationViewController.swift

- Calculates the grouping property of each message, determining
  whether it's first, middle, or last within a sequence either
  sent or received.

- Adjusts the top and bottom constraints for each message bubble
  according to its sequencing.

- Fixes the scroll-to-bottom feature by updating the bottoOffset
  value before conditionally scrolling, removing the scroll
  animation, and only scrolling down when the user is near the
  end of the chat.

- Applies a message bubble grouping style, adjusts line spacing
  of the content, and decouples the theming of the message
  bubble and text colors.

Change-Id: I9118c2bbca0433573c877450c73bd6dc5c9229a0
Reviewed-by: Kateryna Kostiuk <kateryna.kostiuk@savoirfairelinux.com>