This PR adds the first use of Uppy in our codebase, hidden behind a enable_experimental_image_uploader site setting. When the setting is enabled only the user card background uploader will use the new uppy-image-uploader component added in this PR.
I've introduced an UppyUpload mixin that has feature parity with the existing Upload mixin, and improves it slightly to deal with multiple/single file distinctions and validations better. For now, this just supports the XHRUpload plugin for uppy, which keeps our existing POST to /uploads.json.
This PR adds uppy to the project with a custom JS build and the shims needed to import it into our JS code. We need a custom build of Uppy because we do not use webpack for our JS modules/build. The only way to get what you want from Uppy is to use the webpack modules or to include the entire Uppy project including all plugins in a single JS file. This way we can just use the plugins we actually want. Future PRs will actually use Uppy!
Martin Brennan