Long username breaks the markup

Long username (>17 chars) brakes the markup in a topic.

Steps to reproduce:
1. Create an account with username lenght <18 chars
2. Log in
3. Create a topic
4. Log out
5. Create an account with username lenght of 18-25 chars
6. Log in
7. Open the created topic
8. Leave a message

Actual result: Width of the post with long username is different from others
Expected result: Long username is truncated to fit the markup