return sitemap items in newest first order

[eviljeff]

sitemap items are sorted by last modified but they're all returning oldest first currently

[AlexandraMoga]

Sitemap items are now sorted in descending order - most recent item appears first in the list:

Example: https://addons-dev.allizom.org/sitemap.xml?section=collections&debug

[First item]

<url>
    <loc>https://addons-dev.allizom.org/en-US/firefox/collections/4757633/featured-personas/</loc>
    <lastmod>2021-04-13</lastmod>
    <changefreq>daily</changefreq>
    <priority>0.5</priority>
</url>

[Last item]

<url>
    <loc>https://addons-dev.allizom.org/en-US/firefox/collections/4757633/fe/</loc>
    <lastmod>2012-02-11</lastmod>
    <changefreq>daily</changefreq>
    <priority>0.5</priority>
</url>