Provide real "last-modified" header for the topic pages

There is a general good practice to provide fair (real) "last-modified" header for the web-site pages, especially for the forum engines. Most search engines will display provided date near the link in order to inform user about the age of the post. Also, this will decrease amount of unneeded requests by the crawlers.

My proposition is to provide last-modified header at least for the topic pages.

To achieve this, we should do the following:

1. Add "last-modified" response header (and meta tag) to all topic pages, which will always contain last modified date/time (including time-zone) based on the date/time of the last post for the particular topic.
2. Add specific logic to return 304 HTTP code if requested topic page was not modified since date, specified in "If-Modified-Since" request header.

Additional information:

• http://www.feedthebot.com/ifmodified.html
• http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
• http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html