Code cleanup and backwards compatibility for previous messages format #154

Job	Run time
test-postgres-elasticsearch7 (3.8, Django>=4.2,<4.3, false)	7m 32s
test-sqlite (3.10, Django>=4.2,<4.3)	11m 47s
test-mysql (3.9, Django>=4.2,<4.3, false)	17m 44s
test-postgres (3.8, Django>=4.2,<4.3, false, --parallel)	7m 20s
test-mysql (3.12, Django>=5.0,<5.1, false, --parallel, mysql:8.1)	13m 53s
test-postgres (3.11, Django>=5.0,<5.1, false, emailuser, --parallel)	8m 30s
test-sqlite-elasticsearch8 (3.11, Django>=5.0,<5.1, emailuser)	5m 48s
test-sqlite-opensearch2 (3.9, Django>=4.2,<4.3, false)	5m 31s
test-postgres (3.12, Django>=5.0,<5.1, postgres:15, notz, false, --parallel)	10m 23s
test-postgres (3.11, git+https://github.com/django/django.git@stable/5.1.x#egg=Django, postgres:1...	15m 20s
test-postgres (3.12, git+https://github.com/django/django.git@main#egg=Django, true, postgres:lat...	9m 52s
coverage	52s
	1h 54m 32s

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