Fixes #14002

It now accounts for models that have changed but have already been
persisted, such as when called from an UpdateService.

Closes #1773

Fixes #1778

Except for Note, which still overrides it.

There's a lot of code in the Note model that only deals with creating
system notes, so we're going to split that into its own class.

Supports four different event types all bundled under the "note" event type:

- comments on a commit
- comments on an issue
- comments on a merge request
- comments on a code snippet

Resolves #1673.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Add a note to merge requests and issues when they're mentioned by a
merge request, issue, or commit in another project.

Store both the project and identifier of extracted references.  This
prevents `ReferenceExtractor` from returning objects in the wrong
project for cross-project references.