Make username field case-insensitive

**Is your feature request related to a problem? Please describe.**
Now it is case-sensitive. My username is ¨KirillAksenov¨, and I can only login with this one. If I try ¨Kirillaksenov¨ or ¨kirillaksenov¨, I get an error. That could confuse the participants
**Describe the solution you'd like**
"Enter Participant Username" field is case-insensitive
