[openstreetmap-website] Avoid unnecessary array allocations (#140)

[John Firebaugh]

In case it's not obvious: every use of `Array#+=` is equivalent to `array = array + other`, and `array + other` generates a new Array object, leaving the old one to become garbage collected.

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/pull/140#issuecomment-9651791
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