#5315

Driving this change with tests had some pain points. We decided to work
outside-in, starting with an ATC integration test. It was difficult to
move from the outer TDD loop to the inner - I was surprised by the
number of seemingly-far-flung changes that were required to move from
one failure to the next. These included:

* adding an entry to the auditor to overcome a panic
* modifying the `atc/api/present` package to link the DB entity to the
  API entity

both of these things feel so easy to forget, and it might be nice if
they could be described by a change in the same part of the codebase.

Signed-off-by: Jamie Klassen <cklassen@pivotal.io>
Co-authored-by: Bishoy Youssef <byoussef@pivotal.io>
Co-authored-by: Aidan Oldershaw <aoldershaw@pivotal.io>

#5315 - as per
#5346 (comment),
the ability to archive pipelines is now behind an api flag
`enable-archive-pipeline`.

Signed-off-by: Aidan Oldershaw <aoldershaw@pivotal.io>
Co-authored-by: James Thomson <jthomson@pivotal.io>

Not sure how that got in there!

Signed-off-by: Aidan Oldershaw <aoldershaw@pivotal.io>

Specifically for the pipelines/:pipeline_name/archive endpoint

Signed-off-by: Bishoy Youssef <byoussef@pivotal.io>
Co-authored-by: Taylor Silva <tsilva@pivotal.io>

#5316

Rather than adding a concrete error type to check against, we elected to
create an interface for our domain-specific error to satisfy - see:

https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully

Following the example of trigger-job we decided that 409 Conflict was the
most logical http status to use to represent this kind of policy
violation.

We decided to add the `Conflict` interface to the `db` package because
it seems there is no more-appropriate "business logic" or "policy" layer
to place this kind of validation error. Since multiple backend members
in the web node interact with pipelines directly via the database, it
seems justified to place the business logic as close to that integration
boundary as possible.

Recent painful experiences with observability motivated the choice to
treat logging more like a feature, and have unit tests that specifically
require it.

Signed-off-by: Aidan Oldershaw <aoldershaw@pivotal.io>
Co-authored-by: Jamie Klassen <cklassen@pivotal.io>
Signed-off-by: Aidan Oldershaw <aoldershaw@pivotal.io>