Ask to restart GDK after update

As one of the most common actions to perform after updating a GDK is to restart it, by default ask the user if they wish to perform a gdk restart after a gdk update is performed:

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Would you like to restart your GDK?

Any running sidekiq jobs (CI, Geo, etc) may be
interrupted if you say yes.
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Restart GDK (will timeout after 5 secs) [y/N]?

gdk:ask_to_restart_after_update can be set to false in the gdk.yml to stop being prompted if this behaviour is undesirable.

Closes #727 (closed)