0.33.0 Production Release

Scheduled for release to production by 2019-09-25 due to changes to search schema that require reindexing all of our data

User-facing changes include:

• Add extension search parameter, so consumers can filter by file type (e.g. jpg, gif, png...) #360
• Expose dimensions in search results when available #359
• Validate emails supplied during OAuth2 registration with a confirmation email. Existing API key holders will not be affected (except for clearly fake or garbled registrations, which will be ruthlessly purged). #361