frontend: Current mission should be persistent on flight mode

[ArturoManzoli]

Current behaviour

While monitoring a vehicle on a mission, if the browser is refreshed or Cockpit restarted, the current mission is lost, and have to be re-downloaded from the vehicle. But on many situations this is not possible.

Expected or desired behaviour

Keeping a mission copy on cache every time a mission is downloaded, and asking the user if it should be restored would solve the issue.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.