Javascript does not support the negative lookbehind assertion (?<!) used
in the Ruby regex (to disallow usernames ending in `.git` or `.atom`.

Getting the client side code to fully support this format is
non-trivial, since we'd either have to heavily complicate the
regex used, or modify the frontend code to support more complex
validation schemes (it currently uses HTML5 validations).

The pragmatic choice is to create a
`Gitlab::Regex::NAMESPACE_REGEX_STR_SIMPLE` regex to serve as a
Javascript-compatible version of `NAMESPACE_REGEX_STR`.

The client-side code will not display an error for usernames ending in
`.git` and `.atom`, but these will be caught by the server-side
validation.