Update `releases-tab` to use API to fetch releases count #2221

<img width="138" alt="Screenshot 2019-07-10 at 18 33 13" src="https://user-images.githubusercontent.com/1402241/60962594-304c0180-a341-11e9-9da0-687d962ad0de.png">

Currently we only update the releases count if you visit the repo homepage, but now we have 2 good things since this was implemented:

• we use the v4 API
• we have proper caching

Help wanted to change the updateReleasesCount function to be a 1-day cached v4 API call.