Fixed profile caching

[hlbmtc]

Currently, if you edit your username or any other profile data, those changes are only reflected on your own profile page. Other users still see the outdated version because we cache the entire /users/:id endpoint, including all User attributes.

• This PR refactors profile caching so that only slow-to-compute stats are cached. The actual user info is no longer cached and is fetched directly from the database.
• It also replaces the serialize_profile function with two smaller ones: serialize_metaculus_stats and serialize_user_stats.
• Improved get_authoring_stats_data comments count query performance

[elisescu]

LGTM

[lsabor]

looks good to me. I don't see any changes to data generation, just the cache :)