[Embargo] - Enable embargo group selection

The plugin's documentation mentions that when multiple groups are configured, there is supposed to be a dialogue to select the embargo groups.  https://bloomreach-forge.github.io/embargo/screenshots.html 

This is true ONLY for users that belong to the admin group. When a non-admin user belongs to multiple embargo groups and embargoes a document all the groups he belongs are assigned to the document automatically.

In some cases that might not be desired. The requirement for this improvement is to provide a select groups dialogue for users that belong to multiple embargo groups similar to an admin user.