Add Janus custom actions to plugins page

[schultzp2020]

Add the following custom actions to the plugins page:

• SonarQube
• Regex
• Quay
• Kubernetes

References: #206

[schultzp2020]

@christophe-f we need to figure out a name, description, and icon for the above custom actions.

[christophe-f]

Here are the names and descriptions

• SonarQube Custom Action - Creates a new project in SonarQube
• Regex Custom Action - Transforms complex strings in your Golden Path Templates.
• Quay Custom Action - Creates a new repository name in Quay
• Kubernetes Custom Action - Creates a new Kubernetes namespace

I don't think we have/need icons for custom actions. If we need icons, we can add them later.