Setting to enable experimental features

Fixes #569 (closed)

Also hiding custom workflow input form, as well as workflow server interaction unless the user has enabled experimental features.

How to Test

• Go to settings page and confirm that experimental features are disabled by default
• Go to create a server and confirm that there is not an input section for workflow
• Go back to settings page and enable experimental features
• Go to create a server and confirm that there is an input section for workflow
• Launch a server with a workflow
• Confirm that you see the workflow server interaction on the server details page
• Go back to settings page and disable experimental features
• Go back to the server details view for the server with the workflow
• Confirm that you do not see the workflow server interaction

Screenshots

QA Checklist

• Any changes to 'widgets' in src/Style/Widgets/? If so:
  • Update src/Style/StyleGuide.elm showing example usage of that widget
• Were any app flags created, modified, or removed? If so:
  • Update config.js and all files in environment-configs/, including docker-config.js
  • Update "Runtime configuration options" documentation in README.md

Post-Merge Checklist

• Create follow-up issues for anything left over for another merge request
  • If any technical debt is introduced, assign these follow-up issues to the MR submitter