Add unique username for users

[Palakkgoyal]

This PR closes issue no. #544
Approach

• Created a new collection name username
• Add the username as the doc id and the data of the document is the uid of user
• Check if the username is valid or not. Username must only include letters, numbers and _
• If valid username then, check if it already exist.
• If not then let the user create a new id on dummygram using their username.

[narayan954]

looks good to me!

[narayan954]

Thanks for your contribution! :)