Bug #6974 - Hitting Cancel button on New Org > Manually Assign page goes...

... to 404 page
OK, I get the following trace:
```
NoMethodError
undefined method `link_to_back' for #<#<Class:0x000000078ac7d8>:0x0000000722e5a0>
app/views/hosts/_assign_hosts.html.erb:38:in `block (2 levels) in _app_views_hosts__assign_hosts_html_erb__3957567925500005721_42723900'
app/views/hosts/_assign_hosts.html.erb:37:in `block in _app_views_hosts__assign_hosts_html_erb__3957567925500005721_42723900'
app/helpers/layout_helper.rb:250:in `form_for'
app/views/hosts/_assign_hosts.html.erb:5:in `_app_views_hosts__assign_hosts_html_erb__3957567925500005721_42723900'
app/views/hosts/assign_hosts.html.erb:3:in `_app_views_hosts_assign_hosts_html_erb__1850536344537136393_51305740'
app/controllers/concerns/foreman/controller/taxonomies_controller.rb:139:in `assign_hosts'
app/models/concerns/foreman/thread_session.rb:33:in `clear_thread'
lib/middleware/catch_json_parse_errors.rb:9:in `call' 
```

Using `link_to_back(_("Cancel"), { :class=>'btn btn-default' })`