Allow a channel to have a user scope

Background

See INTEGRATED-1037 Closed

Requirements

• Add a new select box field when creating or editing a channel (at /admin/channel/my_channel/edit and /admin/channel/new).
• The new field must be placed below field 'Domains'.
• The new field has a label: 'User scope'
• The select box contains: all user scopes that have been created ( INTEGRATED-1037 Closed ) + the option "No user login allowed". "No user login allowed" is the default value (scope Integrated is not available).
• Store the user scope with the channel.
• Implement the test scenarios (kind of) in Behat

Test scenarios

• Go to /admin/channel/new and /admin/channel/my_channel/edit and check if the select box:
  • has label 'User scope'.
  • is added to the page under field 'Domains'.
  • contains the following options:
    • all user scopes that have been created ( INTEGRATED-1037 Closed ) ,
    • the option "No user login allowed".
  • has a default option: "No user login allowed".
• Check if the chosen 'User scope' is stored within the new or edited channel.