realm: Remove Google Hangouts integration.

Google  has removed the Google Hangouts brand, thus we are removing
them as video chat provider option.
This commit removes Google Hangouts integration and make a migration
that sets all realms that are using Hangouts as their video chat
provider to the default, jitsi.

With changes by tabbott to improve the overall video call documentation.

Fixes: #15298.

zerver/migrations/0285_remove_realm_google_hangouts_domain.py

@@ -0,0 +1,43 @@
+# Generated by Django 2.2.13 on 2020-06-14 01:58
+
+from django.db import migrations
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+VIDEO_CHAT_PROVIDERS = {
+    'jitsi_meet': {
+        'name': "Jitsi Meet",
+        'id': 1,
+    },
+    'google_hangouts': {
+        'name': "Google Hangouts",
+        'id': 2,
+    },
+}
+
+def remove_google_hangouts_provider(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    # We are removing the Google Hangout integration because Google has
+    # removed the Hangouts brand. All the realms that used Hangouts as
+    # their video chat provided are now setted to the default, jitsi.
+    Realm = apps.get_model('zerver', 'Realm')
+    Realm.objects.filter(video_chat_provider=VIDEO_CHAT_PROVIDERS['google_hangouts']['id']).update(
+        video_chat_provider=VIDEO_CHAT_PROVIDERS['jitsi_meet']['id']
+    )
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0284_convert_realm_admins_to_realm_owners'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='realm',
+            name='google_hangouts_domain',
+        ),
+        migrations.RunPython(
+            remove_google_hangouts_provider,
+            reverse_code=migrations.RunPython.noop,
+            elidable=True
+        ),
+    ]