imce 8.x-1.6

Changes since 8.x-1.5:

* Added preview support for svg files.
* Issue #2893614: Allow svg files to be selected as images.
* Issue #2887398 by woprrr: Missing Schema introduced by admin_theme feature
* Issue #2357871 by akalata: Permit IMCE to use the Administration Theme
* Issue #2881119 by Munavijayalakshmi: Use short array syntax.
* Issue #2856229: Manually converted urls to string to prevent a fatal error.
* Issue #2856229 by rocket.man: Replaced deprecated Drupal::url() with Drupal\Core\Url::fromRoute()
* Issue #2865156: Updated description of ckeditor upload checkbox.
* Updated CKEditor integration instructions.
* Removed linear-gradients from some of the UI elements.
* Fixed invalid css property of .imce-tbb
* Revised folder toggle icon.
* Issue #2876870 by rackberg: Fixed incorrect usage of Config::get()