When displaying the pipelines of a project we now preload the following
data:

1. Authors of the commits that belong to these pipelines
2. The number of warnings per pipeline, which is used by
   Ci::Pipeline#has_warnings?

== Commit Authors

Previously this data was queried for every Commit separately, leading to
20 SQL queries being executed in the worst case. With an average of 3 to
5 milliseconds per SQL query this could result in 100 milliseconds being
spent in _just_ getting Commit authors.

To preload this data Commit#author now uses BatchLoader (through
Commit#lazy_author), and a separate module
Gitlab::Ci::Pipeline::Preloader is used to ensure all authors are loaded
before they are used.

== Number of warnings

This changes Ci::Pipeline#has_warnings? so it supports preloading of the
number of warnings per pipeline. This removes the need for executing a
COUNT(*) query for every pipeline just to see if it has any warnings or
not.

[Rails5] Add `touch_later` to `Commit` model

See merge request gitlab-org/gitlab-ce!18642

This commit fixes errors like:

```
1) API::Todos GET /todos when unauthenticated returns authentication error
    Failure/Error: @raw.__send__(method, *args, &block) # rubocop:disable GitlabSecurity/PublicSend

    NoMethodError:
      undefined method `touch_later' for #<Gitlab::Git::Commit:0x00005573f5196270>
    # ./app/models/commit.rb:259:in `method_missing'
    # ./spec/requests/api/todos_spec.rb:12:in `block (2 levels) in <top (required)>'
```

We saw about 10,000 SQL queries for some commits in the NewNoteWorker,
which stalled the Sidekiq queue for other new notes. The notification
service took up to 8 minutes to process the commits. Avoiding this
N+1 query brings the time down significantly.

Closes #45526

Closes #23460

Closes gitaly#1046