chore(rxjs): use pipeable operators instead of prototype-patched based operators

[christophercr]

PR Checklist

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• The commit message follows our guidelines: https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[X] Code style update (formatting, local variables)
[X] Refactoring (no functional changes, no api changes)
[X] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Prototype-patched based observables/operators are used in all our codebase due to this kind of imports:

import "rxjs/add/observable/of";
import "rxjs/add/operator/foo";
import "rxjs/add/operator/baz";

Issue Number: #258

What is the new behavior?

Pipeable observables/operators are used now by doing this kind of imports:

import { of } from "rxjs/observable/of";
import { foo } from "rxjs/operators/foo";
import { baz } from "rxjs/operators/baz";

Currently, we cannot use the generic import for all operators: import { foo, baz } from "rxjs/operators";
due to ReactiveX/rxjs#2981

Does this PR introduce a breaking change?

[X] Yes
[ ] No

Now only pipeable operators should be used, otherwise Tslint will throw an error (due to the "no-import-side-effect" rule)