Add Downloads per Day for Themes

[Luehrsen]

Downloads per day are a rather expensive metric to get, but might be worth it. The plan:

• Keep a daily record of downloads per day per theme
• Aggregate these values daily towards: Avg 7, Avg 30, trend direction

The endpoint to crawl:

https://api.wordpress.org/stats/themes/1.0/downloads.php?slug=twentytwentyfour

ToDo:

• Extend the Theme entity to register DPD_AVG_28, DPD_AVG_7, DPD_LAST_CRAWL
• Register a new Theme_DPD entity to save date, theme, downloads, avg28, avg7
• Expose these values to the frontend