Stick rebranding tour to the hamburger icon on mobile

[tomalec]

Changes proposed in this Pull Request:

Set reference element for rebranding tour to main menu hamburger icon on mobile viewports.

Fixes #2680

Replace this with a good description of your changes & reasoning.

Screenshots:

Before

After

Detailed test instructions:

1. Checkout and build this branch
2. Restart the rebranding tour wp.data.dispatch( 'wc/gla' ).upsertTour( { id: 'rebranding-tour', checked: false } )
3. Open any G4W on a mobile viewport
4. Check that the tour step is visible and dismissable with x button

Additional details:

Changelog entry

Fix rebranding tour on mobile

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.4%. Comparing base (16df5ec) to head (e15ffd0).
Report is 1167 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##             develop   #2692      +/-   ##
============================================
- Coverage       64.9%   62.4%    -2.5%     
============================================
  Files            793     321     -472     
  Lines          24342    5130   -19212     
  Branches        1231    1242      +11     
============================================
- Hits           15790    3201   -12589     
+ Misses          8379    1756    -6623     
  Partials         173     173              

Flag	Coverage Δ
js-unit-tests	62.4% <ø> (-0.3%)	⬇️
php-unit-tests	?

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see 481 files with indirect coverage changes

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[eason9487]

Thanks for the fix! LGTM.