Create reusable pipeline for our webapp project release process

[rhysyngsun]

User Story

We want to implement the new release process we've detailed in our handbook in Concourse, ideally with reusable pipeline(s).

Description/Context

The pipeline should follow this workflow:

flowchart TB
    subgraph main
        sources_main{{main branch}}
    end

    subgraph rc
        direction LR
        sources_rc{{release-candidate branch}}
        subgraph commit [commit to release-candidate]
            direction TB
            vb(version bump)
            changelog(update changelog)
        end
        pr(Pull Request)
        sources_rc --> vb --> changelog --> sources_rc --> pr
    end

    subgraph production
        sources_release{{release branch}}
    end

    main -- New Release Candidate --> rc
    rc -- Release to Production --> production

Acceptance Criteria

• [ ] As an engineer I have a pipeline in Concourse where I can see the current state of the release process.
• [ ] As an engineer I can start a new release candidate
• [ ] As an engineer I can complete a release candidate and promote it to production

Out of Scope

• Slackbot functionality, including commands and messages about the release status