fix: fill slack page background (#687)

Signed-off-by: savisaluwadana <savisaluwadana@gmail.com>

Author: savisaluwadana

src/pages/slack.module.css

@@ -1,4 +1,5 @@
 .page {
+  flex: 1 0 auto;
   padding: 2.75rem 0 5rem;
   background: #f5f6f7;
 }