1. Fix the user extend modal to show the proper number of days they can
   extend.

2. Fix the admin extend modal warning when the extension would violate
   max extension, it was not showing. Add new alerts when we cannot get
   max extension from the cluster or no extension at all allowed.

3. Reduce number of days in the box to max allowed. Warn loudly if you
   type a different number and its greater then max extension.

4. Add "force" box to override. Use with caution. Added the plumbing
   through to the back end as new force option to RenewSliver().

5. Add check in RenewSliver() to ask the reservation system if extension
   allowed before doing it. This was missing, should solve some of the
   over book problems.