Swapped Flex to homegrown component

No more element prop warnings!

frontend/components/PublishingInfo/PublishingInfo.js

@@ -3,7 +3,7 @@ import React, { Component } from 'react';
 import moment from 'moment';
 import styled from 'styled-components';
 import type { User } from 'types';
-import { Flex } from 'reflexbox';
+import Flex from 'components/Flex';
 
 const Container = styled(Flex)`
   justify-content: space-between;