Require username/namespace to be at least 2 characters long

This will allow to keep one letter routes for service needs. Also it should improve autocomplete quality.

Also most of 1 and 2 character length usernames are usually taken by bots.

I suggest we start by adding Rails model validation. Require of minimum 2 characters on username and group path/name.

This is about restricting username and group slug. They are used in URL building.