Add index for username in uppercase

This should speed-up case-insensitive queries
at least on postgreSQL.

Some benchnmarking:

 - with cache
    *  cold cache
        ```
        >>> import timeit
        >>> timeit.timeit("User.objects.get_anonymous_user()", number=100, setup="from inyoka.portal.user import User; from django.conf import settings")
        1.2246114060000082
        ```

    * warm cache (mind the additional 0)
        ```
        >>> import timeit
        >>> timeit.timeit("User.objects.get_anonymous_user()", number=1000, setup="from inyoka.portal.user import User; from django.conf import settings")
        0.29570037799999227
        ```

 - without index (before applying the migration of this commit)
        ```
        >>> import timeit
        >>> timeit.timeit("User.objects.get(username__iexact=settings.ANONYMOUS_USER_NAME)", number=1000, setup="from inyoka.portal.user import User; from django.conf import settings")
        51.55617682000002
        ```
 - with index

        ```
        >>> import timeit
        >>> timeit.timeit("User.objects.get(username__iexact=settings.ANONYMOUS_USER_NAME)", number=1000, setup="from inyoka.portal.user import User; from django.conf import settings")
        0.9243776870002876
        ```


This can be also seen in the EXPLAIN (mind the filter instead of get, otherwise an explain is not possible)

```
>>> print(User.objects.filter(username__iexact=settings.ANONYMOUS_USER_NAME).explain(verbose=True))
Bitmap Heap Scan on public.portal_user  (cost=18.18..2544.97 rows=743 width=447)
  Output: id, password, last_login, is_superuser, username, email, status, date_joined, banned_until, avatar, jabber, signature, location, gpgkey, website, launchpad, settings, forum_read_status, member_title, icon
  Recheck Cond: (upper((portal_user.username)::text) = 'ANONYMOUS'::text)
  ->  Bitmap Index Scan on upper_username_idx  (cost=0.00..17.99 rows=743 width=0)
        Index Cond: (upper((portal_user.username)::text) = 'ANONYMOUS'::text)
```

inyoka/portal/migrations/0036_user_upper_username_idx.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.2.23 on 2024-02-05 21:55
+
+from django.db import migrations, models
+import django.db.models.functions.text
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('portal', '0035_auto_20231127_0114'),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name='user',
+            index=models.Index(django.db.models.functions.text.Upper('username'), name='upper_username_idx'),
+        ),
+    ]

inyoka/portal/user.py

@@ -24,6 +24,7 @@
 from django.core.cache import cache
 from django.core.exceptions import ValidationError
 from django.db import models, transaction
+from django.db.models.functions import Upper
 from django.dispatch import receiver
 from django.utils.html import escape
 from django.utils.translation import gettext as _
@@ -547,6 +548,9 @@ class Meta:
         permissions = (
             ('subscribe_user', 'Can subscribe Users'),
         )
+        indexes = [
+            models.Index(Upper('username'), name='upper_username_idx'),
+        ]
 
 
 class UserPage(models.Model):