Avoid importing custom user models at load time in wagtail.admin.models #159

Job	Run time
test-postgres-elasticsearch7 (3.9, Django>=4.2,<4.3, false)	9m 40s
test-sqlite (3.11, Django>=5.0,<5.1)	13m 28s
test-mysql (3.10, Django>=4.2,<4.3, false)	20m 25s
test-sqlite (3.12, Django>=5.1,<5.2, emailuser, --parallel)	7m 43s
test-postgres (3.9, Django>=4.2,<4.3, false, --parallel)	8m 9s
test-mysql (3.12, Django>=5.1,<5.2, false, --parallel, mysql:8.1)	13m 14s
test-postgres (3.12, Django>=5.0,<5.1, postgres:15, notz, false, --parallel)	10m 40s
test-sqlite-elasticsearch8 (3.12, Django>=5.1,<5.2, emailuser)	9m 27s
test-postgres (3.11, git+https://github.com/django/django.git@stable/5.1.x#egg=Django, postgres:1...	16m 17s
test-sqlite-opensearch2 (3.10, Django>=5.0,<5.1, false)	8m 23s
test-postgres (3.12, git+https://github.com/django/django.git@main#egg=Django, true, postgres:lat...	10m 12s
coverage	54s
	2h 8m 32s

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