Add plain text alternative to email messages #163

Job	Run time
Run PEP8 code style checks	21s
pytest-integration / check-img-cache	6s
pytest-unit / check-img-cache	5s
pytest-unit / test-img-build / build-image	1m 48s
pytest-integration / test-img-build / build-image	1m 58s
pytest-unit / run-tests	9s
pytest-unit / test-img-build / scan-image	0s
pytest-integration / test-img-build / scan-image	0s
pytest-integration / run-tests	8s
	4m 35s

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