Track/store if the user is available for sharing.

Just for Chrome so far.
Bundle description + sharing in GlobalSettings obj
Remove unused update functions.
Add button to toggle sharing state.
Merge branch 'dev' of https://github.com/uProxy/uproxy into lucyhe-add_stop_sharing_state
Send instance update when global settings update.
Change uProxy extension and app names to 'uProxy, part 1 of 2' and 'uProxy, part 2 of 2'
Merge with dev
Remove "Stop Sharing" code.
Remove reference to sharing status.
Merge with origin/dev.
Remove Core.GlobalSettingsCommand.
Remove white space; edit comments.
Save custom STUN servers to storage.

- STUN config now updated with updateGlobalSettings and not
setStunServer.
Remove old setStunServer code.
Refactor network and initial state sync
Merge manifest changes
Merge remote-tracking branch 'origin/dev' into lucyhe-add_global_settings
Remove outdated TODOs.
Merge pull request #632 from uProxy/lucyhe-add_global_settings

Bundled 'description' and custom STUN servers into global settings object.