Use eslint config from the js sdk

Extend the js sdk's eslint config to give as consistent a code
style as possible. Add react/jsx/flow stuff that we use here.

.eslintrc.js

@@ -0,0 +1,62 @@
+module.exports = {
+    parser: "babel-eslint",
+    extends: ["./node_modules/matrix-js-sdk/.eslintrc.js"],
+    plugins: [
+      "react",
+      "flowtype",
+    ],
+    env: {
+        es6: true,
+    },
+    parserOptions: {
+        ecmaFeatures: {
+            jsx: true,
+        }
+    },
+    rules: {
+        /** react **/
+        // This just uses the react plugin to help eslint known when
+        // variables have been used in JSX
+        "react/jsx-uses-vars": "error",
+
+        // bind or arrow function in props causes performance issues
+        "react/jsx-no-bind": ["error", {
+            "ignoreRefs": true,
+        }],
+        "react/jsx-key": ["error"],
+
+        /** flowtype **/
+        "flowtype/require-parameter-type": ["warn", {
+            "excludeArrowFunctions": true,
+        }],
+        "flowtype/define-flow-type": "warn",
+        "flowtype/require-return-type": ["warn",
+            "always",
+            {
+              "annotateUndefined": "never",
+              "excludeArrowFunctions": true,
+            }
+        ],
+        "flowtype/space-after-type-colon": ["warn", "always"],
+        "flowtype/space-before-type-colon": ["warn", "never"],
+
+        /*
+         * things that are errors in the js-sdk config that the current
+         * code does not adhere to, turned down to warn
+         */
+        "max-len": ["warn"],
+        "valid-jsdoc": ["warn"],
+        "new-cap": ["warn"],
+        "key-spacing": ["warn"],
+        "arrow-parens": ["warn"],
+        "prefer-const": ["warn"],
+
+        // crashes currently: https://github.com/eslint/eslint/issues/6274
+        "generator-star-spacing": "off",
+    },
+    settings: {
+        flowtype: {
+            onlyFilesWithFlowAnnotation: true
+        },
+    },
+};