Public URL to User

As a writer I'd like to have a nice looking URL to my articles and profile so that I can share it with others and my name can be recognizable there

Additional reason for this is that user's nick name can contain special symbols that would be hard to escape in URL.

Acceptance Criteria:

• I can edit my profile and there I'll have a field Public URL antarcticle.org/users/ and I can specify there my 'public name'. After specifying something there, URL to my profile will be: antarcticle.org/users/mypublicname
• User can't specify the name that is already taken by someone else.
• User can't use symbols that will cause troubles during creation of URL
• User can change the name any amount of times she wants
• On pages where URL leads to user's profile:
  • If user didn't specify her public name, URL should look like antarcticle.org/users/id/11234
  • If user specified her public name, link should look like: antarcticle.org/users/publicname. URLs with IDs still should be working.