! Add new "Max list items" setting and improve consistency in some areas...

... (fixes #2028)

Long explanation:
1. Add and implement a new "max list items per page" setting. This is used in most places where a standard list is used, unless another setting applies (such as for lists of members). Previously some lists used $modSettings['defaultMaxMessages'] while others had hard-coded values for the "items per page" option.
2. Attempt to use the user-selectable "messages per page" and "topics per page" options everywhere they might apply rather than just in the message index or while viewing a topic. Main exception here is the search results page since that's covered by a different setting.

Signed-off-by: Michael Eshom <oldiesmann@oldiesmann.us>