[FIX] owrank: Add migrate_settings

[ales-erjavec]

Issue

ref #1794

Fix an error when restoring from a workflow that was saved before widget supported ranking features on data with no class.

Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/canvas/scheme/widgetsscheme.py", line 454, in create_widget_instance
    widget.__init__()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/owrank.py", line 260, in __init__
    if self.headerState[2] is not None:
IndexError: tuple index out of range

Description of changes

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Current coverage is 88.96% (diff: 100%)

Merging #1797 into master will not change coverage

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  Files            82         82          
  Lines          8963       8963          
  Methods           0          0          
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  Hits           7974       7974          
  Misses          989        989          
  Partials          0          0          

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