Djiffy

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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

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* Update for pytest 5.x and mysql change on Travis-CI

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* Fix pytest version requirement for failing build on Travis-CI

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* New method on Manifest object to get text label for rights license

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* Fix a git merge issue that resulted in some code not being merged into master