settings: Add support for customizing the top-left logo.

This adds a new realm_logo field, which is a horizontal-format logo to
be displayed in the top-left corner of the webapp, and any other
places where we might want a wide-format branding of the organization.

Tweaked significantly by tabbott to rebase, fix styling, etc.

Fixing the styling of this feature's loading indicator caused me to
notice the loading indicator for the realm_icon feature was also ugly,
so I fixed that too.

Fixes #7995.

zerver/lib/actions.py

@@ -43,6 +43,7 @@ from zerver.lib.message import (
     update_first_visible_message_id,
 )
 from zerver.lib.realm_icon import realm_icon_url
+from zerver.lib.realm_logo import realm_logo_url
 from zerver.lib.retention import move_messages_to_archive
 from zerver.lib.send_email import send_email, FromAddress, send_email_to_admins
 from zerver.lib.stream_subscription import (
@@ -3086,6 +3087,22 @@ def do_change_icon_source(realm: Realm, icon_source: str, log: bool=True) -> Non
                               icon_url=realm_icon_url(realm))),
                active_user_ids(realm.id))
 
+def do_change_logo_source(realm: Realm, logo_source: str) -> None:
+    realm.logo_source = logo_source
+    realm.logo_version += 1
+    realm.save(update_fields=["logo_source", "logo_version"])
+
+    RealmAuditLog.objects.create(event_type=RealmAuditLog.REALM_LOGO_CHANGED,
+                                 realm=realm, event_time=timezone_now())
+
+    send_event(realm,
+               dict(type='realm',
+                    op='update_dict',
+                    property="logo",
+                    data=dict(logo_source=realm.logo_source,
+                              logo_url=realm_logo_url(realm))),
+               active_user_ids(realm.id))
+
 def do_change_plan_type(realm: Realm, plan_type: int) -> None:
     old_value = realm.plan_type
     realm.plan_type = plan_type