Fixes #11223 & #11248 - Reindex app label

[Alef-Burzmali]

Fixes: #11223 and #11248

Add the possibility to reindex all the models of an app_label via the management command reindex.

The second commit modifies the reindex call in migration extras.0083_search to reindex only the models of core NetBox, to prevent plugins from blocking the migration as in #11248. Plugins should add their own reindex migration when they add a SearchIndex, such as:

import sys

from django.core import management
from django.db import migrations

def reindex(apps, schema_editor):
    # Build the search index (except during tests)
    if "test" not in sys.argv:
        management.call_command("reindex", "<plugin_label>")

class Migration(migrations.Migration):
    dependencies = [
        ("<plugin_label>", "<NNNN_previous_migration>"),
    ]

    operations = [
        migrations.RunPython(
            code=reindex, reverse_code=migrations.RunPython.noop
        ),
    ]

[jeremystretch]

Thanks for taking this on @Alef-Burzmali!