* Consolidate context menu actions into one class used by Stories list and my stories
* add context menu to story viewer
* change hidden stories header into a regular cell
* generate context menu at tap time to properly reload hidden state
* some cleanup
* pr feedback
* Add DelegatingContextMenuButton to maintain desired hooks without deviating ContextMenuButton's API from that of UIButton
* Start muted, unmute when pressing volume buttons
* move ringer switch observation into its own class
* observe ringer switch in stories
* add foreground time to AppContext; use to drive mute foregrounding behavior
* dont double observe
* mix story volume with others, show volume controls
