Username duplicate determination

[kittybest]

Describe the bug
If a username is used by me before and I changed mine to another username, the previous username should be used by others or me again.

To Reproduce
Steps to reproduce the behavior:

1. create account A
2. set username to kitty1
3. set username to kitty2
4. set username to kitty1 again
5. should display error: username already exists

Expected behavior
Reading the code in contract, should release username after changing it.

Screenshots

[ChialiT]

Won't fix due to sunsetting the project

[kittybest]

I have a different opinion, I think this problem still exists if we are still going to do social media kind of app.

[ChialiT]

reopen issue for us to remind us how to build better in next one.

[vivianjeng]

I think this could be solved by letting user reveal the epoch key nonce.
I will write it in v2 tests.