[angular] Polish, add tests and improve automatic language change logic

[kaidohallik]

Uses similar style for SessionStorageService and LocalStorageService variable names:

• using sessionStorageService and localStorageService everywhere
• previously also sessionStorage, $sessionStorage, localStorage and $localStorage was used, names $sessionStorage and $localStorage are probably AngularJs relicts

Adds more tests to AccountService.

Improves automatic language change logic:

• previously when navigating away and back to application then chosen language was reapplied only if valid authentication existed
• now previously chosen language is reapplied also if user is not authenticated

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• All continuous integration tests are green
• Tests are added where necessary
• jhipster-online was updated if necessary
• Documentation is added/updated where necessary
• Coding Rules & Commit Guidelines as per our CONTRIBUTING.md document are followed

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[DanielFran]

LGTM