system groups - UI - allow user to change the env/view for systems in a group

This PR contains modifications to allow a user to change (via UI) the environment and content view for all users in a system group.  This PR also contains changes to the group's system list to show the current env/view associated with each system.

To change the env/view, the user would:
- go to System Groups -> <group A> -> Systems
- click the 'Edit' link
- select environment/content view and click 'Save'

Since this will change the values for all systems in the group, the user is presented with a confirmation dialog before proceeding with the change.

As part of these changes, I created 'env_content_view_selector.js' which can hopefully be reused for similar scenarios in the UI (e.g. Systems & Activation Keys create).  I've also done some minor refactoring of those other areas to centralized the definition of some strings and view helpers.  (Will do any further refactoring to use this common js as part of a separate PR.)