Fixes various street and neighborhood API bugs

[misaugstad]

Resolves #3032
Resolves #974

This fixes a few bugs introduced by our recent PRs in the /v2/access/score/streets and /v2/access/score/neighborhoods APIs, particularly on streets with no label data.

Here's a mostly comprehensive list of bugs I found. All of these are new as of the most recent version.

• There were cases where we were returning duplicate attributes in the attributes and attributesWithLabels endpoints
• If there are no labels on a street/neighborhood, we were returning an incorrect timestamp. Now we are just returning null
• If a street or neighborhood had no labels, sometimes there would just be a server error
• For the streets API, we were computing the average image and label age based on any type of label on the street, whereas AccessScore only uses our four primary label types. We now only include the four primary label types in this calculation.

In addition, instead of saying whether a street has been audited, we now say how many audits the street has had (there are zero audits if it is unaudited).

And finally, I cleaned up the code to hopefully prevent further mistakes in the API, mostly by reusing more code!

Before/After screenshots (if applicable)

Before (streets API)

After

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