Refactor server switching in the controller

[data-sync-user]

Currently, server switching (silent or user-initiated) results in sending a deactivate signal to the platform-specific controller, followed by an activate signal. However, for all platforms (except possibly flatpaks), the deactivate is a no-op. This is confusing, and has tripped up many people interpreting this code. Additionally, using the activate signal for a config switch (to a new tunnel) is responsible for an entire class of bugs.

We should add a new controller function for server switches, streamlining this code.

┆Issue is synchronized with this Jira Task
┆Reporter: Matt Cleinman