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* pulled out all modal overrides into a general `modal-overrides` file + cleanup of outdated CSS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`
* UX: separate invite-signup styling
* UX: invite page centering
* remove old invites-show css
* UX: invite signup page – mobile
* remove class references in general file
* add styling for instructions
