Implement limit for /new

As suggested by @noffle in %lzkFJa2....
The default limit is 500, and it can be changed by setting SSB config option patchfoo.newLimit. It can also be overridden with a query string parameter limit. When /new is loading messages, if it reaches the limit, it load no more messages, and shows a message at the top of the page saying it reached the limit. Then you can process the messages and click catchup as usual and it will show more messages.
This prevents patchfoo from attempting to load huge amounts of messages into memory after a big sync.

cc: @cryptix @kas @Giarc

re: %B+UWzuj... %Br+AT+r...

Use Post/Redirect/Get for all message publishing

Intended to prevent accidentally publishing a duplicate message or being required to republish a message to refresh the page.

Requested by @tohl (thread: %wiojbU3... ).