fix(popover): added margin to arrow placement
fix(popover): added margin to arrow placement

Fixes #1845
Closes #1855

ng-bootstrap/ng-bootstrap,jnizet/ng-bootstrap,ng-bootstrap/core,ktriek/ng-bootstrap,benouat/ng-bootstrap,chenyuzhcy/ng-bootstrap,pkozlowski-opensource/core,ng-bootstrap/ng-bootstrap,ACGC/ng-bootstrap,jnizet/ng-bootstrap,ng-bootstrap/core,fbasso/ng-bootstrap,mschoudry/ng-bootstrap,jnizet/ng-bootstrap,maxokorokov/core,mschoudry/ng-bootstrap,fbasso/ng-bootstrap,ACGC/ng-bootstrap,fbasso/ng-bootstrap,benouat/ng-bootstrap,maxokorokov/core,Foxandxss/ng-bootstrap,fbasso/ng-bootstrap,benouat/ng-bootstrap,ng-bootstrap/ng-bootstrap,ng-bootstrap/ng-bootstrap,ktriek/ng-bootstrap,ktriek/ng-bootstrap,maxokorokov/core,pkozlowski-opensource/core,Foxandxss/ng-bootstrap,maxokorokov/core,jnizet/ng-bootstrap,ktriek/ng-bootstrap,benouat/ng-bootstrap,Foxandxss/ng-bootstrap,ACGC/ng-bootstrap,mschoudry/ng-bootstrap,chenyuzhcy/ng-bootstrap,pkozlowski-opensource/core,ng-bootstrap/core,chenyuzhcy/ng-bootstrap