Add method to navigate to the last page of various SHOW endpoints

Brought up on Discord before, primarily with the desire to be able to always be able to go to the last page of a particular topic no matter what via a bookmark. This idea could also be usefully applied to several other indexes where the count of items is already known, such as favorite groups and pools.

This could be done by making a brand new endpoint, but appending something like /last to the end of the SHOW endpoint. Another way would be to use ?page=last .

Example:

• /forum_topics/1234/last
• /forum_topics/1234?page=last