Refactor rxjs imports and operator usage (use `pipe`)

[harunurhan]

Instead of import 'rxjs/add/operator...'which does prototype patches Observable, Subject etc., do import { ... } from 'rxjs/operators';. Then compose them, via pipe operator.

https://blog.angularindepth.com/rxjs-understanding-lettable-operators-fe74dda186d3

[mattmutt]

would recommend moving to rxjs 6 and validating the component against current Angular at some point. There will be necessary library updates to make as well, such as ngx-bootstrap. Currently one would have to patch with a temporary shim library like rxjs-compat. Ideally the right import syntax and pipe operators would fix all of this.