Ensure "Add Interface" form in Admin Panel works

This commit add new templates for interface add/delete operations
in admin panel to make sure the original page is displayed after
adding a router interface successfully.

It also changes the following:
- Update the help message in "Add Interface" and "Set Gateway" forms
  (both in project and admin panels)
- Fixes a issue that Project router detail was displayed after setting
  an external gateway to a router in admin panel.
  Admin router detail is expected to display
- Make sure a router name is displayed in router detail in admin panel.

Fixes Bug #1131177

Change-Id: I9728cb68972977f868549b61979d9c779cd9fd4e
Merge "Ensure "Add Interface" form in Admin Panel works"