Djiffy

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* Manifest dynamic properties for license uris and labels are now cached
* Manifest license logic now handles CreativeCommons license URIs.
* Images for rightstatement.org and some CC licenses are now included in static content
* License image path can be generated using the `license_image` property for Manifests with supported licenses

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* `ManifestImporter.import_paths` now returns a list of database manifest
objects for the requested uris, whether newly imported or already available
* Setup GitHub Actions workflow for sphinx documentation

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* Include manifest attribution information in ``extra_data`` on import

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* Improved error handling for connection errors when importing IIIF manifests

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* Now tested against Django 3.1 and 3.2; dropped support for Django 1.11 and 2.2
* Now tested on python 3.7, 3.8; dropped support for python 3.5 and 3.6
* Shifted continuous integration from Travis-CI to GitHub Actions
* Admin thumbnail format is now configurable in django settings via **DJIFFY_THUMBNAIL_FORMAT**; default is png
* Manifest import check for "supported" manifests can be disabled by setting **DJIFFY_IMPORT_CHECK_SUPPORTED** to False in django settings
* Canvas data now includes dimensions, with convenience properties `width` and `height`

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* Now supports Django 1.11 through 3.0; dropped support for Django 1.10