This commit makes it so the fullscreen code modal grows
to fit its content, and doesn't show horizontal scrollbars
unless the entire screen is filled by the modal already.
The code syntax highlighting and copy buttons were also
broken in fullscreen because of modal changes over time.
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* pulled out all modal overrides into a general `modal-overrides` file + cleanup of outdated CSS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`
