Merge remote-tracking branch 'Mavlink/master' into MavlinkLogUploader

* Mavlink/master:
  Only run unit tests on pull requests
  MAVLink: Update to most recent spec
  PX4 param and airframe metadata update (#4169)
  Don't apply throttle deadband in ThrottleModeDownZero
  Use FTDI driver provided by ftdicjip.com
  AppImage update to libsdl2 (#4155)
  Minor formatting fix
  Added a second polygon. This polygon is updated from the first polygon at each mouse click and is what will ultimately be saved upon completion of drawing.
  Fixed a bug where a black line would be rendered when starting to draw a polygon.
  Fix brand image visibility calculation
  Remove FileManager unit test
  VTOL text change
  Update VTOL command text
  Update message
  Temp removal of Onboard File
  Changing label
  Making deadbands stupid-proof; general cleanup
  Cleanup & Sync
  Implementing throttle axis accumulator with deadband support
  Handle redirects in file downloads