feat: Extract user account from common

[lafeychine]

As requested by @jocelynj, extracting account creation from common to create new accounts simpler.

The following changes had been made:

• Extract users creation into a new role users;
• Add the new playbook accounts.yml to create a user (the playbook common.yml behaves in the same way as before);
• Create an "all" group_vars with a dictionary of every user;
• Update syntax to Ansible 2 (Remove warnings).

The following regressions are:

• Breaking change: The shell /bin/bash is not set up during the first creation.

  The removed complex logic to check if an account exists is necessary for this fact, as the shell is not set up any more.
  In fact, the ansible.builtin.user Ansible module overrides any value changed by the user, and provides no way to leave it untouched.
  Since every machine is running on Debian, the shell is in any case /bin/bash by default.

• Duplicating .gitconfig and .vimrc, files.

  The roles/users/files location is the expected location for these files.
  However, there are still tasks (shared/project-account.yml and roles/common/tasks/ssh-tunnel.yml) that seem redundant with this new users role, but I need some guidance before making any changes.

As this is my first PR on the project, feel free to tell me if I need to change anything.

[lafeychine]

The force-pushed branch is an unsquash from the work done.