[angular] Angular fixes and polishing, noticed while creating test refactor branch

[kaidohallik]

While creating test refactor branch https://github.com/kaidohallik/generator-jhipster/tree/refactor-angular-tests noticed some problems. This PR fixes them. Contains 4 commits:

• Follow up to [angular] Reduce event manager importance and usage #13111 which forgot to add catch block to Promise in delete dialog close which caused console error in delete rejection. Now replaced Promise with Observable (available from ng-bootstrap v8). Added comment about why not unsubscribing to give confidence that no memory leak here.

• Follow up to ng-jhipster migration to generator-jhipster #13022 which put metrics blocks into metrics root folder, it makes metrics folder difficult to read. Now fixed that by moving blocks into blocks subfolder.

• Add missing license headers.

• Polish imports.

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[mshima]

LGTM