Nickname prefix wraps on certain view widths

**Describe the bug**

On the new registration page, the nickname prefix moves to a separate line.

**To Reproduce**

Steps to reproduce the behavior:

1. Go to '/users/sign_up'

2. Resize viewport

3. See the nickname input wrapping to a new line

**Expected behavior**

Prefix and input stay on the same line

**Extra data (please complete the following information):**

 - Device: Desktop

 - Device OS: Ubuntu 18.04

 - Browser: Chrome, Firefox

 - Decidim Version: 0.21