Continued work on moderate flags UI.
In this PR admins are allowed to change the order of flags. The notify user flag is always on top but all other flags can be moved.
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* pulled out all modal overrides into a general `modal-overrides` file + cleanup of outdated CSS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
Krzysztof Kotlarek
Kris
Sam
chapoi
Penar Musaraj
Meghna
Alan Guo Xiang Tan
Jarek Radosz