Crls/feat/show manager username and counts in catalog admin

[ccantillo]

Show counts and manager usernames in PartnerCatalog admin list

Summary

Improves the PartnerCatalog change list in Django admin so that admins
can see at a glance how many learners and courses each catalog has, and
who is managing it — without leaving the list view.

---

What changed

Column behaviour

The Add Learner, Add Course, and Add Manager columns now work
on two levels:

┌─────────────────────────────────────────────────────────────────┐
│  Column header (clickable)  →  opens a blank add form           │
│  Cell value   (clickable)   →  opens the add form pre-filled    │
│                                with the catalog already selected │
└─────────────────────────────────────────────────────────────────┘

Element	What you see	Where it goes
Column header	Add Learner / Add Course / Add Manager link	Generic add form — no catalog pre-selected
Cell value	Learner count · Course count · Manager username(s)	Add form with the catalog field already filled in

Visual flow:

 ┌──────────────┬─────────────┬──────────────┐
 │ [Add Learner]│ [Add Course]│ [Add Manager]│  ← header links (blank form)
 ├──────────────┼─────────────┼──────────────┤
 │     [42]     │    [10]     │   [jsmith]   │  ← cell links  (pre-filled)
 │     [0]      │    [5]      │     [—]      │
 └──────────────┴─────────────┴──────────────┘

• Learner and course cells show the count as the link text.
• Manager cells show the active manager's username as the link text,
  or — if no active manager is assigned.
• Multiple active managers are listed one per line.

---

Why a custom admin template was added

Django admin assigns CSS classes to table cells automatically
(.field-<method_name> for <td>, .column-<method_name> for <th>),
but there is no way to set text-align on those cells from inside the
cell's own HTML. Any inline style placed on the <a> or wrapper element
inside the cell only affects that inner element — it cannot reach the
<td> or <th> container.

To center the column headers and cell values without touching global
admin CSS, we added a minimal template override:

partner_catalog/templates/admin/partner_catalog/partnercatalog/change_list.html

This file extends Django's stock admin/change_list.html and injects a
small <style> block scoped to #result_list that targets only the
three affected columns:

#result_list .column-add_learner,
#result_list .column-add_course,
#result_list .column-add_manager,
#result_list .field-add_learner,
#result_list .field-add_course,
#result_list .field-add_manager {
  text-align: center;
}

The template is placed in the app's own templates/ directory and is
scoped to the partnercatalog change list only, so it has no effect on
any other admin view.