Exhibition dates, IIIF manifest updates, unit testing and doc updates

• Use public dates from website for exhibition dates [WEB-1822]
• Remove deprecated fields from exhibitions endpoint [WEB-1827]
• Add link to artwork on website to IIIF manifests [WEB-1809]
• Also serve IIIF manifests at the path /manifest [WEB-1809]
• Add /manifest.json to documentation [WEB-1809]
• Add PHP CodeSniffer package to codebase and Travis builds [WEB-1823]
• Refactor unit tests to run in local file-based SQLite database [WEB-1801]
• Clean up tests and factories [WEB-1801]
• Update references to aggregator-data.artic.edu to api.artic.edu [WEB-1428]
• Add missing anchor to GET and POST links in documentation
• Update tip and warning box colors in documentation to match AIC branding
• Correct suggested syntax for IIIF URLs in docs
• Add database index to v1_artwork_term