UX: refactor .d-modal to use BEM and improve styling (#23967)

This PR refactors the following:
* leaving all the CSS applied to the old `modal-body` classes in their respective files
* made  new clean styling for `.d-modal` and refactored the template to use the new BEM classes
  * `inner-`, `middle-`, `outer-` container classes are gone and replaced with simplified `wrapper` and `container` classes  
  * use standardised max-sizes with modifiers `-large` and `-max`
  * lighter backdrop,
  * min-width to prevent puny modals
  * other styling changes regarding padding, close button,…
* pulled out all modal overrides into a general `modal-overrides` file + cleanup of outdated CSS
* pulled out login and create account modal styling into their own file, cause it's such a big override 
* removed old general login.scss file for mobile & desktop
* only kept some remainders I don't want to touch in `app/assets/stylesheets/common/base/login.scss`

spec/system/page_objects/modals/badge.rb

@@ -16,7 +16,7 @@ module PageObjects
       end
 
       def grant
-        within(modal) { find(".modal-footer .btn").click }
+        within(modal) { find(".d-modal__footer .btn").click }
       end
 
       def has_success_flash_visible?