Add a feed detail/permalink page for feed stories

Summary:
Ref T2852. Asana has one bug which I'm having a little trouble figuring out. I want to get more information to debug it, but I'll need them to run `bin/feed republish <story_id>` to get that data.

Right now, it's incredibly hard to figure out the story ID for feed stories. So mostly this is to make that easier (click permalink; pull it out of the URL), but it also adds a little functionality and cleans the code up a bit.

The page itself could be prettier and maybe some day we'll add comments or whatever, but it seems reasonably functionalish.

Test Plan:
{F49962}

  - Also loaded many pages of feed history to check that nothing broke.

Reviewers: btrahan, chad

Reviewed By: chad

CC: chad, aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6440