Add support for Sentry Releases functionality

Sentry are currently working on some [interesting release-based functionality](https://docs.sentry.io/learn/releases/), which could help us with determining what changes are responsible for new bugs, and communicating quickly and non-noisily with the responsible developers.

*Update*: this is now released https://blog.sentry.io/2017/05/01/release-commits.html

Regarding the support we should add to [Prout](https://www.theguardian.com/info/developer-blog/2015/feb/03/prout-is-your-pull-request-out), the Sentry API has a [_Create a New Release for a Project_](https://docs.sentry.io/api/releases/post-project-releases/) call, which we would get Prout to call every time a PR is seen on Prod. Prout would provide a direct link to the GitHub PR to provide context - there doesn't appear to be any other way to provide human-readable context in the Sentry release at the moment, except possibly in the `version` field. Ideally we'd like the Sentry _Release Overview_ page to display to the PR number & title - data we could provide with Prout, if there was a legitimate field in Sentry to store it.

Some of the listed features of Sentry's release work:

* Learn which release an issue was first introduced or last seen in
* Suggested owners on issues
* Marking issues as resolved in the next release
* Resolving Sentry issues via commit messages

I've already [connected](https://docs.sentry.io/integrations/github/) the [membership-frontend](https://github.com/guardian/membership-frontend) repository in Sentry, and hopefully with the next few released PR's we'll see some increased metadata in the entries under https://sentry.io/the-guardian/membership/releases/


cc @jacobwinch, and also @macqueen, who is working on this :sparkles: awesome stuff at Sentry :sparkles:

Start with Prout's handling of a pull request being seen on Prod and review Sentry's Create a New Release for a Project API. The work is done when each such PR creates a Sentry release containing a usable version and direct GitHub PR context, with the Release Overview showing the PR number and title if Sentry supports that field.

Written by the indexing model from the issue text.