Add #out_of_range? method to PageProxy

enhancement for #599.
Refactor action view helper methods

 * if statement + 1 method is easier to read than unless statement + 2
   methods.
 * Put #prenset? for code consistency
Do not display link to previous page if scope is out of range
:flushed:
prev_page method doesn't not have to be too intelligent

We mightn't like this behaiviour as it behaves differently from
Paginator::PageProxy.