refactor(datepicker): simplify disabled state handling
refactor(datepicker): simplify disabled state handling

Fixes #1829

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