[rss] Add publication time to the rss_feeds table.
[rss] Add publication time to the rss_feeds table.

Adding this before the actual code that needs it, because we are migrating
to new table anyway. This can be used later to detect when an entry has been
removed or modified.

Uname-a/knife_scraper,Uname-a/knife_scraper,Uname-a/knife_scraper