admin view for user imports

- makes user_import_time_limit a site setting rather than a value in settings.py (note this applies to exports as well as imports)
- admins can change user_import_time_limit from UI
- admins can cancel stuck user imports
- disabling new imports also disables user imports

bookwyrm/migrations/0184_sitesettings_user_import_time_limit.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.20 on 2023-10-22 02:27
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('bookwyrm', '0183_auto_20231021_2050'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='sitesettings',
+            name='user_import_time_limit',
+            field=models.IntegerField(default=48),
+        ),
+    ]