Remove `with_username` completely

https://assembly.com/coderwall/bounties/479

This PR replaces the `with_username` function with `find_by_username` when it searches only by the internal  username (coderwall username), and with `from_elsewhere_username` when it searches by an external username. 


> can we think about another method name ?

Sure, `find_by_external_username`?

> provider is not optional in this case.

True, let me change that.