Refactored the SignalApp class to Swift
Fixed a horizontal alignment issue in the ConversationTitleView
Fixed an issue where expiration timer update messages weren't migrated or rendering correctly
Fixed an issue where expiring messages weren't migrated correctly
Fixed an issue where closed groups which had been left were causing migration failures (due to data incorrectly being assumed to be required)
Shifted the Legacy Attachment types into the 'SMKLegacy' namespace
Moved all of the NSCoding logic for the TSMessage
