add Google auth plugin

plugins/talk-plugin-google-auth/client/components/GoogleButton.css

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+.button {
+  background-color: #db3236;
+  border-color: #db3236;
+  color: rgb(255, 255, 255);
+  width: 100%;
+  box-sizing: border-box;
+  padding: 10px 20px;
+}
+
+.button:hover {
+  background-color: #c71e22;
+  border-color: #c71e22;
+}

plugins/talk-plugin-google-auth/client/components/GoogleButton.js

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+import React from 'react';
+import { BareButton } from 'plugin-api/beta/client/components/ui';
+import styles from './GoogleButton.css';
+
+export default ({ onClick, children }) => {
+  return (
+    <BareButton className={styles.button} onClick={onClick}>
+      {children}
+    </BareButton>
+  );
+};

plugins/talk-plugin-google-auth/client/components/SignIn.js

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+import React from 'react';
+import GoogleButton from '../containers/GoogleButton';
+import { t } from 'plugin-api/beta/client/services';
+
+export default () => {
+  return (
+    <GoogleButton>{t('talk-plugin-google-auth.sign_in')}</GoogleButton>
+  );
+};

plugins/talk-plugin-google-auth/client/components/SignUp.js

@@ -0,0 +1,9 @@
+import React from 'react';
+import GoogleButton from '../containers/GoogleButton';
+import { t } from 'plugin-api/beta/client/services';
+
+export default () => {
+  return (
+    <GoogleButton>{t('talk-plugin-google-auth.sign_up')}</GoogleButton>
+  );
+};