Fix #3899 & New Bug: Team name verification failure and meaningless prompts

I have fixed issue #3899 and found another related bug. I am working on the new bugs.

**Observed Behaviour**

- An error message "This field is required" appears when we add special characters in the form field of "Create a new Team", which is unclear and doesn't make sense. (As issue in [Add meaningful error message on form validation #3899](https://github.com/Cloud-CV/EvalAI/issues/3899))
- No verification for “Update Host Team”, which means that special characters can appear.


**Expected Behaviour**

- The error message should be more precise, for example, "Team Name is invalid".
- Perform verification at both frontend and backend.

**Steps To Reproduce**
- On the home page, click on "Host Teams".
- Input some special characters (Such as @,[,{ etc.) in the "Create a new team" form fields on the right
- Click on "Create Host Team"
- Click on the pencil icon on the left fields of "Select a Challenge Host Team".
- Change the team name and then submit.
- You will find that illegal characters for team name present.