centraldashboard: Support dynamic logout URL

[orfeas-k]

Use dynamic URL for the logout button that can be set by the LOGOUT_URL ENV variable in the deployment of the app. If it is unset, a default '/logout' string is passed from the backend to the frontend.

I documented in the issue here details on how I dealt with binding data between Polymer elements.

During this PR, we also had to fix a test that was broken. It seems that the test was failing because new Date () added whitespace that was not equal to the space character during comparison. Instead of comparing objects of type Date and string in the expect statement, we went with comparing Date objects in order to ensure comparison in a uniform and type-strict way.

[kimwnasptd]

Thanks for the PR @orfeas-k! Could you send the commit for fixing the tests as a dedicated PR?

Looks like we are having some build issues when PRs are merged and I'd like to tackle this as a dedicated effort
74f020e
65e41bf

[orfeas-k]

Sure, I also removed the tests commit from this PR

[kimwnasptd]

@orfeas-k tested the PR and while the logout works, I end up seeing a blank screen. Navigating back to the root URL gets me to login, so the logout functionality works, but I think something might not be configured correctly on the AuthService's side.

I'll look more into it with @athamark. @DomFleischmann there's a chance we'll need some fix for this, but we'll comment back once we have more news

[kimwnasptd]

I'll go on and merge this for now to include this in the RC1

/lgtm
/approve

[google-oss-prow]

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