Add support for error logging via Sentry

[denschub]

We did not catch https://github.com/diaspora/diaspora-project-site/pull/125#issuecomment-428214928 at all, and since this a pretty important asset for the project, it probably makes sense to add some kind of logging and alerting to our site...

Internal note:

• I will create accounts on my Sentry instance for the core team as soon as this is merged.
• I already set up the production env, so that deploying this will automatically start sending errors.