added ability to create and return rentals

-Creating a rental will create new rental and update equipment status
to properly reflect availability
-Equipment that is not available will not display as an option when
creating a new rental
-Returning a rental will do the same as above but in reverse. Will
mark equipment as available, update rental status and return equipment
to the rental poole.
-Also updated nav to pass appropriate GET request to list users/rentals/
equipment when landing on page. (the blank pages left to much guesswork for the
user)