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* pulled out all modal overrides into a general `modal-overrides` file + cleanup of outdated CSS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`
This PR adds a new "Pause notifications" checkbox to the user status modal. This checkbox allows enabling the Do-Not-Disturb mode together with user status. Note that we don't remove and don't rename the existing DnD menu item in this PR, so the old way of entering the DnD mode is still available.
Also, we're not making DnD mode a part of user status on backend and in database. The reason is that the DnD mode should still be available on sites with disabled user status, having them separated helps keep the implementation simple.
