Refresh entity list after updating custom group (self_hook_*)

Refresh entity list after updating custom group (self_hook_*)

Overview
----------------------------------------

The CustomGroups are used to generate virtual entities (if they set `is_multiple`). Toggling `is_multiple`
should cause the virtual entity to be added or removed.

Reproduction steps
----------------------------------------

1. Create a custom data group. Leave default `is_multiple=0`.
2. View "API Explorer". Look for the virtual entity. (It's not there. Properly so.)
3. Edit custom data group. Change to `is_multiple=1`.
4. Reload "API Explorer". Look for the virtual entity.

Before
----------------------------------------

The entity does not appear. You have to do a system-flush.

After
----------------------------------------

The entity does appear.