Support for Custom UI Plugins

[kristenmills]

General idea of how this can work:

If users want custom plugins, those files will be served directly from a custom ui directory. Those files can be injected as script tags on page load. Those files are required to call a function we provide on window to register themselves as plugins.

In execution there is a little more to discuss but that's the basic idea.

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Update 8/22/20: I think taking advantage of webpack federation will work pretty well here. Then we don't have to worry about issues such as multiple version react running on the pages and other similar issues.