User-Onboarding: Random username generation + Unique usernames

[mzparacha]

Link to Issue

Closes: https://github.com/hicommonwealth/commonwealth/issues/6751

Description of Changes

• Added random username generation
• Added username uniqueness checks

"How We Fixed It"

Test Plan

• Enable FLAG_USER_ONBOARDING_ENABLED in .env.
• Add type='create-account' to <AuthModal/> instance in SubLayoutHeader.tsx or apply this diff

  
  diff --git a/packages/commonwealth/client/scripts/views/components/SublayoutHeader/SublayoutHeader.tsx b/packages/commonwealth/client/scripts/views/components/SublayoutHeader/SublayoutHeader.tsx
  index 4564d26340..c58647b700 100644
  --- a/packages/commonwealth/client/scripts/views/components/SublayoutHeader/SublayoutHeader.tsx
  +++ b/packages/commonwealth/client/scripts/views/components/SublayoutHeader/SublayoutHeader.tsx
  @@ -59,6 +59,7 @@ export const SublayoutHeader = ({
       open={isFeedbackModalOpen}
     />
     <AuthModal

• type='create-account' onClose={() => setIsAuthModalOpen(false)} isOpen={isAuthModalOpen} />

open the modal and create a new web3 wallet account, verify you see the `Welcome to Common` modal after a new account is created.
- Verify that you can generate a random username with the `Generate random username` button
- Verify that the generated or user entered username is validated as unique, if its not unique there will be a `Username already exists` error below the username input

## Deployment Plan
N/A

## Other Considerations
N/A

[mzparacha]

yarn.lock is causing some CI issues for some reason. I tried creating a new yarn.lock with

node=20.10.0
yarn=1.22.22

but it caused nearly all the CI tests to fail. Unsure about the resolution, will update PR once the issue is found.

[Israellund]

Getting this error when I try to run yarn start. Just see a blank white screen

[mzparacha]

@Israellund can you try again?