Add a system to backup/sync saved builds

[atomicptr]

While that feature generally needs an overhaul, it might be interesting to expand this.

Possible solutions:

• Offer a backend for dauntless-builder.com with it's own account system. Not sure how I feel about this, one reason why I chose to use github pages and the fully client-side approach was to keep costs low because tons of community tools get shut down over time because the developer doesn't want to pay for said tool anymore. Which in this case is solved by hosting on Github pages...
• Hijack something like dropbox/google drive to store the data
• Firebase?