Fix CSS Grid for Pro channel tiles

Summary

Fixed an issue that was making the Pro channel media tiles to go beyond the screen width.

Closes #2358 (closed).

Steps to test

• Log in with a Pro channel, ensure your Pro channel is set to "published" in settings
• Upload at least 3 videos with a very long post text (more than 300 characters).
• Go to your channel
• Mark 2 of the video posts as pinned (using the ribbon icon at the top right)
• Log out, login with another user
• Visit the Pro channel, it should not overflow like in #2358 (closed) video

Estimated Regression Scope

Pro channels video/image/blog tiles would look awkward by not occupying around the 50% of the screen viewport.