Fix POST handling of localized PNI products

[Pomax]

Closes #7502

This gives PNI products an .original_product accessor for finding the English version of a product (based on product slug), with a refactor of the code that fetches the DEFAULT_LOCALE and DEFAULT_LOCALE_ID, which are required to make sure that we can find the original English version of products. We didn't have "one place to get them" so there were a bunch of files that all reinvented the wheel, all of those now use the same function to get default locale information.

This also update some of the @property functions for ProductPage so that total votes and creepiness are derived from the original product, and not a localized alias/published version of that product, both of which are used in the average-creepiness calculation for ranking items on the homepage/category pages.

Finally, it makes sure to always template the creepiness value into the product page in an unlocalized fashion, so that the JS that hooks into it doesn't get confused by numbers that use commas instead of period for decimal points.

Bad case STR:

• be on the main branch,
• run an inv copy-stage-db to grab a copy of the staging database,
• run inv migrate just to make sure the db is at the tip of our migration chain
• run docker-compose up and load up http://localhost:8000/en/privacynotincluded/categories/video-call-apps/ as well as http://localhost:8000/fr/privacynotincluded/categories/video-call-apps/ in two tabs

This will show a different ordering, where on the English page, you should see Threema, Signal, FaceTime, and Jitsi Meet as top four, whereas on the French page, there are very different elements at the top.

• DOM inspect the pages, and you should see a list of <figure...> elements that have data-creepiness that starts at ~13 and ends at ~82 for the English page
• on the French page, you'll see these values start at 0,10660980810234541 (or similar), with comma instead of period, and end at 98,6 (or similar), again with comma.

Fixed case STR:

• docker-compose down
• check out this branch
• docker-compose up
• reload the two category pages
• both should show identical ordering
• inspecting the DOM should show identical data-creepiness values

[Pomax]

this just got moved up, so that it's next to the other vote-related function/properties

[Pomax]

This is basically a noop refactor, I thought I would be able to add tests in this PR, but it turns out that's much more work, so I filed #7559 to tackle that monster.

[TheoChevalier]

Did some testing with the staging DB, couldn’t spot any issue