Add BlocksJsonSerialization model concern and include it in User

Closes #37947

See merge request gitlab-org/gitlab-ce!14350

Previously, this would include the entire User record in the update
hash, which was rendered in the response using `to_json`, erroneously
exposing every attribute of that record, including their (now removed)
private token.

Now we only include the user ID, and perform the lookup on-demand.

If Unlinking a fork would fail somewhere after this, the LFS objects
might still be linked. Which would cause issues when trying to destroy
a project.

Resolve "Todo notification count shows notification without having a todo"

Closes #40871

See merge request gitlab-org/gitlab-ce!15807

When the target is deleted, todos are destroyed, but we did not reset the todo
cache for users with todos on the deleted target. This would only update after
the next time the todo cache was updated for that user.

Replace factory_girl_rails with factory_bot_rails

See merge request gitlab-org/gitlab-ce!15919

Simplify conflict resolver interface

See merge request gitlab-org/gitlab-ce!15897

This does two things:
- Pass commit oids instead of `Gitlab::Git::Commit`s. We only need the
former.
- Depend on only the target repository for conflict listing. For
conflict resolution, treat one repository as a remote one so that we can
implement it as such in Gitaly.

I've followed the [upgrade guide](https://github.com/thoughtbot/factory_bot/blob/4-9-0-stable/UPGRADE_FROM_FACTORY_GIRL.md) and ran these two commands:

```
grep -e FactoryGirl **/*.rake **/*.rb -s -l | xargs sed -i "" "s|FactoryGirl|FactoryBot|"
grep -e factory_girl **/*.rake **/*.rb -s -l | xargs sed -i "" "s|factory_girl|factory_bot|"
```

Signed-off-by: Rémy Coutable <remy@rymai.me>

Clear caches before updating MR diffs

See merge request gitlab-org/gitlab-ce!15916