Fiddle max_length in focal_point_key migrations to make them superficially different

This prevents the migration engine from optimising them away - see https://github.com/wagtail/wagtail/pull/3293#issuecomment-274828807

wagtail/wagtailimages/migrations/0001_initial.py

@@ -63,7 +63,7 @@ class Migration(migrations.Migration):
                 ('file', models.ImageField(width_field='width', upload_to='images', height_field='height')),
                 ('width', models.IntegerField(editable=False)),
                 ('height', models.IntegerField(editable=False)),
-                ('focal_point_key', models.CharField(editable=False, max_length=16, null=True)),
+                ('focal_point_key', models.CharField(editable=False, max_length=18, null=True)),
                 ('filter', models.ForeignKey(on_delete=models.CASCADE, related_name='+', to='wagtailimages.Filter')),
                 ('image', models.ForeignKey(on_delete=models.CASCADE, related_name='renditions', to='wagtailimages.Image')),
             ],

wagtail/wagtailimages/migrations/0004_make_focal_point_key_not_nullable.py

@@ -44,6 +44,6 @@ class Migration(migrations.Migration):
         migrations.AlterField(
             model_name='rendition',
             name='focal_point_key',
-            field=models.CharField(blank=True, default='', max_length=16, editable=False),
+            field=models.CharField(blank=True, default='', max_length=18, editable=False),
         ),
     ]

wagtail/wagtailimages/migrations/0016_deprecate_rendition_filter_relation.py

@@ -35,6 +35,12 @@ class Migration(migrations.Migration):
         #   that successfully applied the old 1.8 version of this migration are consistent with
         #   other setups.
         #
+        # Since Django will optimise away any AlterField operations that appear to match
+        # the current state (according to earlier migrations) - which would cause them to be
+        # skipped on installations that ran the earlier (max_length=255) versions of the
+        # migrations - we need to make them superficially different; we do this by stepping
+        # max_length down from 18 to 17 then 16.
+        #
         # Projects with a custom image model don't have to worry about this - they'll have an existing
         # migration with the max_length=255, and will get a new migration reducing it to max_length=16
         # the next time they run makemigrations.
@@ -42,7 +48,7 @@ class Migration(migrations.Migration):
         migrations.AlterField(
             model_name='rendition',
             name='focal_point_key',
-            field=models.CharField(blank=True, default='', max_length=16, editable=False),
+            field=models.CharField(blank=True, default='', max_length=17, editable=False),
         ),
 
         migrations.AlterUniqueTogether(

wagtail/wagtailimages/migrations/0017_reduce_focal_point_key_max_length.py

@@ -13,9 +13,9 @@ class Migration(migrations.Migration):
     operations = [
         # The Wagtail 1.8 version of migration wagtailimages/0016 did not include the
         # step to reduce focal_point_key's max_length to 16, necessary to make it work
-        # on some MySQL configurations. This migration serves only to ensure that
-        # installations upgrading from 1.8 to >=1.8.1 have this change applied; on other
-        # setups (where the current 0016 and 0017 are applied together), this is a no-op.
+        # on some MySQL configurations. This migration (added in 1.8.1) ensures that
+        # installations that were already successfully running 1.8 receive this change
+        # on upgrading to 1.8.1 or later.
         migrations.AlterField(
             model_name='rendition',
             name='focal_point_key',