Bastion: System Bulk Actions: initial support..

...for system group, content and remove actions

This request contains changes to support the following system
bulk actions:
- Remove Systems
- System Group Add / Remove
- Content (Pkg/Pkg Group/Errata) Install / Update / Remove

Based on prior discussions, these changes are to provide parity with 
the bulk actions provided on old tupane.  It does not address the
longer term work the team has discussed and is planning for 
managing bulk actions.

Additional caveats:
- This does not include tests.  I wanted to get the initial code available for review, while the tests are being written.
- This does not include the 'Auto-attach all systems to available subscriptions'. 
Both of the above will be added in separate commits.