Added index on SidebarCategories table (#20045)

```release-note
NONE
```


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

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

@@ -0,0 +1,14 @@
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'SidebarCategories'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_sidebarcategories_userid_teamid'
+    ) > 0,
+    'DROP INDEX idx_sidebarcategories_userid_teamid on SidebarCategories;',
+    'SELECT 1;'
+));
+
+PREPARE removeIndexIfExists FROM @preparedStatement;
+EXECUTE removeIndexIfExists;
+DEALLOCATE PREPARE removeIndexIfExists;

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

@@ -0,0 +1,14 @@
+SET @preparedStatement = (SELECT IF(
+    (
+        SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
+        WHERE table_name = 'SidebarCategories'
+        AND table_schema = DATABASE()
+        AND index_name = 'idx_sidebarcategories_userid_teamid'
+    ) > 0,
+    'SELECT 1;',
+    'CREATE INDEX idx_sidebarcategories_userid_teamid on SidebarCategories(UserId, TeamId) LOCK=NONE;'
+));
+
+PREPARE createIndexIfNotExists FROM @preparedStatement;
+EXECUTE createIndexIfNotExists;
+DEALLOCATE PREPARE createIndexIfNotExists;

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);