Add toggle settings for transport plugins

This branch adds a setting for enabling or disabling each transport plugin.

Since the Tor plugin already had a setting that could be used to disable the plugin, the existing setting is migrated at startup.

The Bluetooth plugin had a setting that determined whether contact connections were allowed, or only key agreement connections. This setting now has the effect of enabling or disabling the plugin. The key agreement UI is updated to enable the plugin if necessary before adding a contact in person, if the user accepts the Bluetooth discoverability prompt. In other words we never enable the plugin without the user's consent. Once it's been enabled, it will be enabled again at each startup until the user disables it.

The LAN plugin is now turned off by default, and the key agreement UI doesn't turn it on. This may affect the success rate of adding contacts in person. We should ensure that the user testing for this feature covers that scenario.