Reordering custom categories/fields doesn't work unless you've reordered a non-custom field first

[drusepth]

Expected behavior

• Reordering fields/categories should always work.
• If fields/categories don't have an internal sort_order value, they should initialize a sort order and then perform the sort

Actual behavior

It seems that if a user creates a custom field/category and then sorts it, it applies a sort_order value but doesn't always initialize the values for the other fields/categories it's sorting again. Not sure if this is all the time or if it only gets into this bad state in some cases.

The issue can be "fixed" by reordering any other field/category, which initializes sort order for all other fields.

We should be initializing sort order for all categories/fields upon creation of those objects so this never happens.