feat(Challenge Page): A page for community challenges

[TTalex]

What

• See Community challenges ? #934 for description, and questions
• This PR is only used to see code diffs for the prototype, it is not ready for merging

Fixes bug(s)

• Fixes Community challenges ? #934

[teolemon]

@TTalex do you have screenshots of this ?

[raphodn]

@TTalex do you have screenshots of this ?

There is 1 in the related issue (#934)

[TTalex]

Some updates with the upcoming Nutella challenge:

• Added two tutorial cards leading to multiple proof uploader and Price validation assistant
• Changed title, dates, example image and tracked categories to match the new challenge
• Moved most text to translations
• Added stats on proof uploads
• Moved page to experiments
• Make API calls work using feat(Price Stats): Stats grouped by open-prices#675

New Page look:

[TTalex]

Update:

• Removed elements requiring group by stats (Top contributors, user ranking, chart)
• Reverted change on PriceChart, as it is no longer used
• Edit filter name from product__categories_tags__contains to product__categories_tags__contains, which will most likely be the way it's added on backend (see Price API: Allow filter on product categories tags open-prices#673)
• Some mobile friendly UI tweaks

New page look: