Add Client.get_user_named

[Soheab]

Summary

This PR adds a method called get_user_named to Client as an equivalent to Guild.get_member_named.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[lmaotrigine]

Docs build is failing

[Rapptz]

I'm not really sure on the usefulness of this method on whether it's worth adding to the Client class, especially since discriminators aren't really a thing anymore.

[Soheab]

I kept the discriminator impl because of bots and "dead" users I guess. It can be removed all at once anyways.

But not sure where else it could go.. in utils maybe?