Fixed double scroll and page shift when Modal opens.

Solution for #8768. Using only css, as noted by @fat at #9021

Issues:

1. Double scroll is shown when Modal is opening.
This is fixed by enabling scroll for Modal only when body has `.modal-open` class. 

2. Page shifts, because of killing body scroll.
This is fixed by wrapping whole page into `.modal-scrollfix`, which has scroll space when body has no scroll.
Fix also aware about `.navbar-fixed-*`.

Solution works pretty good, but I am not sure is additional wrapper good for Bootstrap or not. So you can decide.