Switch back to legacy decorators

For matrix-org/matrix-react-sdk#3961

Author: turt2live

babel.config.js

@@ -11,7 +11,7 @@ module.exports = {
         "@babel/preset-react"
     ],
     "plugins": [
-        ["@babel/plugin-proposal-decorators", {decoratorsBeforeExport: true}],
+        ["@babel/plugin-proposal-decorators", {legacy: true}],
         "@babel/plugin-proposal-export-default-from",
         "@babel/plugin-proposal-numeric-separator",
         "@babel/plugin-proposal-class-properties",