Implement sorting for search result

[ArtemkaKun]

This PR does 2 things:

• removes nr_downloads field from Package structure
• implements search results sorting logic

Sorting logic is the following:

• by stars (descending)
• if stars count if the same - be downloads count (descending)
• if stars and downloads counts are the same - by the last update date (ascending)
• if stars and downloads count and last upload date are the same - by name (A -> Z)

This PR is unrelated to #92 , since it implements default sorting logic, while an issue requests the implementation of controls to be able to choose sorting mode on the search page.

[xy3]

Can we also keep the data from nr_downloads? I believe it actually does contain some download counts from the jsmod endpoint

[ArtemkaKun]

Migration code of old packages should insert nr_downloads into downloads filed, if I understood a code right. So probably we just need to remigrate packages

[medvednikov]

I'll take care of migrating the downloads numbers.