- Remove unused Gitlab::Application.routes.url_helpers from Ci::Build
- Remove too much logic from a view, use Ci::Commit.matrix_builds
- Use ci_status_with_icon
- Don't describe symbols

- Remove all view related methods from Ci::Build and CommitStatus
- Remove unused Ci::Commit and Ci::Build methods
- Use polymorphism to render different types of CommitStatus

It's unclear how these blank values got added, but GitLab.com had a few:

```
irb(main):002:0> Note.where("line_code IS NOT NULL AND line_code = ''").count
=> 439
```

We've added a migration to convert any existing records to use a NULL
value when blank, and updated Note to set blank values to nil before
validation.

The OR condition for source_project_id/target_project_id leads to a
query plan that performs rather poorly on PostgreSQL due to the use of
sub-queries. Because Rails offers no easy alternative for this
particular problem we're forced to using a UNION for both conditions.
The resulting query performs much faster than just using an OR.

This ensures searching namespaces works exactly the same as searching
for any other resource.