Describe the bug

What the title says, it seems like you can't use special characters (like Swedish or Spanish accentuated characters) in your alias but this is not mentioned around this field and when trying to Sign Up, the error message is not explicit.

To Reproduce
Steps to reproduce the behavior:

1. Go to the Sign Up page
2. Fill in the form, use a word with special characters for alias (see examples below)
3. Click on Sign Up
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

We should have a short description of what can and can't be used in this field. Something like Your username can only contain letters, numbers and '_', that Twitter is using.

It could be present on the page from the beginning or just appear in red when a special character is detected.

Screenshots
If applicable, add screenshots to help explain your problem.

Extra data (please complete the following information):

• Decidim Version: latest
• Decidim installation: try it on try.decidim.org