Fix user creation with stronger minimum password requirements

## What does this MR do?

`doc/security/password_length_limits.md` Describes a way to increase minimum requirements of passwords, via `config/initializers/devise_password_length.rb`. However, user creation hardcoded password generation at 8 characters.

If the minimum was set at any more than 8 characters, user creation will fail.

Function now looks up minimum length requirement.

## What are the relevant issue numbers?

Fixes #14563.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4054