Epic: Move MDN to new front-end framework (Kuma.Next)

[davidflanagan]

This is an epic for design, architecture and prototyping work toward a next-generation version of Kuma using a modern front-end framework. This is related to Epic #231 (Update front-end development workflow) but is more specifically focused on the adoption of a front-end framework and all the tools and infrastructure that goes with that.

Acceptance criteria

• The decisions and prototyping work described in issues of this epic are complete (or we decided they were unneeded).
• The development team is comfortable with the decisions that were made
• At least two features using the new framework and tools have been deployed

[jwhitlock]

2018 work was assigned to Epic #231