Allow setting `can_create_group`

[joshuaspence]

Currently it's not possible to override this setting:

    ## User settings
    default_projects_limit: <%= @gitlab_projects %>
    # default_can_create_group: false  # default: true
    # default: true - User can change her username/namespace

[sbadia]

sounds a good idea indeed, maybe @alexcern already managed this with #234 ?

[alexcern]

Hi,

Yes, I will include it in #234.