Add missing indices relevant for Repo Deletion

[Swatinem]

There are a few very slow DELETE queries as part of repo deletion. The worst of those is deleting the newly created "shadow owner", which takes >1 minute.

Running all the on_delete hooks using django (which has since been fixes) has revealed that the SET NULL of Commit.author is the culprit, as that column did not have any index previously. That is similar to the SET NULL on Repository.fork, except that query is by far not as slow, probably because of a lot less records that need to be scanned.

---

While at it, this also fixes a django warning related to the default not being a callable, and aligns the Repository.fork SET NULL constraint with the reality of the database.

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The generated SQL for the migration looks like this. Interestingly, altering the fields does not run any actual migration here, as it seems it only has an effect on the django migration state.

--
-- Alter field fork on repository
--
-- (no-op)
--
-- Alter field languages on repository
--
-- (no-op)
--
-- Concurrently create index commits_author_cd2f50_idx on field(s) author of model commit
--
CREATE INDEX CONCURRENTLY "commits_author_cd2f50_idx" ON "commits" ("author");
--
-- Concurrently create index repos_forkid_4cd440_idx on field(s) fork of model repository
--
CREATE INDEX CONCURRENTLY "repos_forkid_4cd440_idx" ON "repos" ("forkid");

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.90%. Comparing base (8db93ad) to head (421097a).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
- Coverage   90.32%   89.90%   -0.42%     
==========================================
  Files         435      324     -111     
  Lines       12999     9114    -3885     
  Branches     2112     1615     -497     
==========================================
- Hits        11741     8194    -3547     
+ Misses       1143      857     -286     
+ Partials      115       63      -52     

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