User nicknames don't check cases when checking for uniqueness

I found these two users:

• https://meta.decidim.org/profiles/carol/activity
• https://meta.decidim.org/profiles/Carol/activity

I thiough they were the same user, but they aren't. The problem here is that the system is not checking the letter cases when checking for the uniqueness.

The bug is found at Metadecidim, so it's present in whatever version Metadecidim is running on.