Refactoring username/password Validation

[ehsan-g]

let's reconsider some of the validation we have at front and back, and make sure we have a consistent validation.
if you decided to make any changes (e.g username minimum char) please let me know to consider in in front-end

[iRhonin]

@ehsan-g, can you please update the title and description?
Also, this issue is pending on the frontend patch.

[iRhonin]

• Changed min length of username from 4 to 6
• username must start with a letter, not with a number

@ehsan-g, @Elhamne
Waiting for frontend implementation to deploy.