Add module priority setter in Horizon

Priority in CK modules is used to set the execution order of all enabled modules.
Currently, the priority value can only be set with the REST API.

This blueprint proposes to add a configuration page in CK Horizon dashboard

Work items:

* In the module admin page, replace "enable/disable" button in the "actions" column by a dropdown list with the following entries:
    * toggle availability: fire toggle between enabled and disabled mode (same as current button)
    * set priority: open dialog box to set integer value as priority for the module
* In the module admin page, add extra column with priority value