Lost scroll position when modal open

[kimsible]

This PR #2962 proposed to disable the scrollbar instead of hide it but it brought one regression, when we open a modal we lose the scroll position :

We have 2 solutions for modals that does not impact the new disabling scrollbar UI :

1. Do not fix the body when modal opened, so the user can still scroll the page
   

2. Back to hide scroll-bar only when modal opened, the user will see the upload button and search input move to the right
   

@rigelk @Chocobozzz what solution do you prefer ? do you have another solution ?

EDIT: I myself prefer the first solution, YouTube uses this ways to display modals (not that YouTube is a great example, but for UI/UX it's a good example)