Restructure frontend code to allow for extensions (#4721)

# Description of Changes
Move frontend code into `core` folder and add infrastructure for
`proprietary` folder to include premium, non-OSS features

frontend/src/core/components/shared/FileUploadButton.tsx

@@ -0,0 +1,46 @@
+import { useRef } from "react";
+import { FileButton, Button } from "@mantine/core";
+import { useTranslation } from "react-i18next";
+
+interface FileUploadButtonProps {
+  file?: File;
+  onChange: (file: File | null) => void;
+  accept?: string;
+  disabled?: boolean;
+  placeholder?: string;
+  variant?: "outline" | "filled" | "light" | "default" | "subtle" | "gradient";
+  fullWidth?: boolean;
+}
+
+const FileUploadButton = ({
+  file,
+  onChange,
+  accept,
+  disabled = false,
+  placeholder,
+  variant = "outline",
+  fullWidth = true
+}: FileUploadButtonProps) => {
+  const { t } = useTranslation();
+  const resetRef = useRef<() => void>(null);
+
+  const defaultPlaceholder = t('chooseFile', 'Choose File');
+
+  return (
+    <FileButton
+      resetRef={resetRef}
+      onChange={onChange}
+      accept={accept}
+      disabled={disabled}
+
+    >
+      {(props) => (
+        <Button {...props} variant={variant} fullWidth={fullWidth} color="blue">
+          {file ? file.name : (placeholder || defaultPlaceholder)}
+        </Button>
+      )}
+    </FileButton>
+  );
+};
+
+export default FileUploadButton;