JKAN should check if updates are available

[timwis]

#51 added a Check for updates button to the /admin/ page that takes you to a pull request page on github.com:

https://github.com/<repo_owner>/<repo_name>/compare/gh-pages...timwis:gh-pages?expand=1
There's an API call for it as well:
https://api.github.com/repos/<repo_owner>/<repo_name>/compare/gh-pages...timwis:gh-pages?expand=1 (example)

We could check if ahead_by is greater than 0 and show the Upgrade button if it is, even an alert when an administrator logs in that updates are available. But how do we check if it can be a clean merge?

Ideally, we'd check to see if there's a new release available. But how does JKAN know what release it's currently running?

[timwis]

Looks like we can check if a new release is available by checking the releases on the current repo and comparing them to the releases on the upstream repo using github's releases api