Gin adds sidebar to modals #2 [#3606443]

Problem/Motivation

The workaround created in https://www.drupal.org/project/sites/issues/3601178 only works when clicking on the SitesSelection modal button. It needs to be adapted to the initial modal load, when adding an entity.

Proposed resolution

Add the part from 3601178 in TriggerOpenSitesSelectionModalController::open(). Maybe create a helper for this?

Issue fork sites-3606443

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1.x changes, plain diff MR !30
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