activation keys / systems - minor refactor to allow for reuse on system groups

For content views, we will be updating system groups to allow
the admin to change the env/view for all systems in a group.
This will use a similar env/view widget to what is currently
used in both activation keys and systems.  In preparation for
that, this commit is moving some of the logic to be common.

This commit primarily addresses:
1. moves the definition of 'no content view' - placing it in common_i18n
   and application_helper
2. moves the logic for obtaining the content view select labels
   to application_helper