Applications: Ship icons and icon list for frontend/backend

[juliusknorr]

• [x] Extract icons and metadata that we pick from https://github.com/Templarian/MaterialDesign
• [x] Add icons and json to the repo
• [x] Frontend: Implement icon picker popover instead of an emoji picker based on the list from the json added in #902
• [x] Backend: Validate icon when saving context, expose url to navigation entries

cat meta.json| jq '.[]|select(.author == "Google" and (.name | contains("-") | not)) | .name | "svg/" + . + ".svg"' -r | xargs -I{} cp {} picked/