Validate taken usernames (front end)

[crchong1]

We recently switched our sign up flow, and right now, we don't have a proper check to see if usernames have been already taken. If you go to CompleteSignupFlow.tsx, you will see where we render all of the signup pages:

If you go to AccountSetup.tsx, you will need to add additional functionality to check if a username exists.

This is the function that checks if the username is invalid. Right now, we only check for forbidden characters and blank fields. However, we need to write a fetch call to the server. It will look like similar to this:

You basically need to send to the server the username field, which will return a boolean response.