Djiffy

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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

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* Canvas now has extra_data field to capture additional manifest information
(currently `rendering`) as part of the import.
* Canvas has `plain_text_url` property to supply url for plain-text
transcription/OCR if available from manifest.

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* Import script now supports updating previously imported manifests;
use `--update` option.
* Import script now allows manifests with viewing hint 'individuals'
as well as paged.
* Manifest object now has properties for logo, license, and
rightstatement.org id
* Canvas short id logic can be customized in ManifestImporter subclasses
* Project app verbose name is now 'IIIF Content' for listing in Django
admin, so it will be more meaningful & recognizable to users
* Minor template improvements
* bugfix: ManifestSelectWidget now handles empty string