Set better HTTP headers for RSS feeds

[r-a-y]

BP's activity RSS feeds need to support ETag and browser cache invalidation to mirror WP's feed support.

[r-a-y]

In 7477:

Set better HTTP headers for activity RSS feeds.

Previously, BP's RSS feeds would use WP's native HTTP headers set in the
WP::send_headers() method. This would cause BuddyPress to inherit WP
feed's "ETag" and HTTP status headers.

When trying to refresh BP's RSS feeds, this would not work correctly due
to BP relying on WP's post last modified date instead of BP's activity last
updated date. This would lead to a HTTP 304 Not Modified status header
being sent by WP, meaning the client (browser, RSS reader) would not fetch
the latest items from the activity stream.

This commit sets the appropriate HTTP headers for BP's activity RSS feeds
and uses a new, internal class method - http_headers() - to set them.

Fixes #5213.