Added index on SidebarCategories table (#20045)

```release-note
NONE
```


Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

db/migrations/postgres/000087_sidebar_categories_index.down.sql

@@ -0,0 +1 @@
+DROP INDEX IF exists idx_sidebarcategories_userid_teamid;

db/migrations/postgres/000087_sidebar_categories_index.up.sql

@@ -0,0 +1 @@
+CREATE INDEX IF NOT EXISTS idx_sidebarcategories_userid_teamid on sidebarcategories (userid, teamid);