Add a local editing mode for the deployed website

[RunDevelopment]

We want to allow people to edit models directly on the deployed website, but we need to build the technical foundation first.

The goal of this issue to provide a local editing mode. Users should be able to enter and exit this editing mode at will. While in editing mode, the website will use an in-memory version of the backend database that it will continuously update as the user makes changes. To then share those changes, it also needs to be able to generate a diff (=which files changed in what way).