Add default_versions database table

[Turbo87]

This PR adds a new database table: default_versions. This table has only two columns: crate_id and version_id.

Once the table has been filled with the update-default-versions admin command it will contain a row for every crate in the database, pointing to the version that will be shown by default by the front-end (not implemented yet) and can be used for other queries where useful (e.g. reverse dependencies).

This calculation has to be performed on the Rust side, since it involves sorting by SemVer and the Postgres extension for SemVer is unfortunately not available on Heroku.

There are four different operations to consider to keep this table in sync:

• Publishing a new crate: we write the version_id straight into the database since this is the first and therefore default version.
• Publishing a new version of an existing crate: we update the default version in a background job. this leaves us with a short time of the table pointing to an older, but existing version.
• Yanking/Unyanking a version: we update the default version in a background job. (see above)
• Deleting a version (admin action): we update the default version within the same transaction that deletes the versions. the foreign key constraint would otherwise prevent us from deleting a version that is currently a default version.

Note that the code currently does not prevent the update_default_version() fn from running multiple times in parallel for the same crate. I'm open to implementing such a lock, I just haven't found a good way to do it yet. It currently does not deduplicate the background jobs either, but since the job is quite fast it is most likely not needed anyway.

Note also that this table is currently write-only and not used anywhere yet. Once we have had this running in production for a few days we can check whether we see any inconsistencies before we use the data anywhere.

[codecov]

Codecov Report

Attention: Patch coverage is 79.20792% with 42 lines in your changes are missing coverage. Please review.

Project coverage is 88.33%. Comparing base (5f069aa) to head (af423fc).

❗ Current head af423fc differs from pull request most recent head fef072e. Consider uploading reports for the commit fef072e to get more accurate results

Files	Patch %	Lines
src/admin/delete_version.rs	0.00%	23 Missing ⚠️
src/admin/update_default_versions.rs	0.00%	16 Missing ⚠️
src/admin/yank_version.rs	0.00%	1 Missing ⚠️
src/bin/crates-admin.rs	0.00%	1 Missing ⚠️
src/schema.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8484      +/-   ##
==========================================
- Coverage   88.44%   88.33%   -0.12%     
==========================================
  Files         270      274       +4     
  Lines       26899    27106     +207     
==========================================
+ Hits        23792    23943     +151     
- Misses       3107     3163      +56     

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