Reply on a feed post comments on the issue or pull it is about, without leaving the feed #8211

The vision says a reply is a comment. Today a feed card's foot (feedCard in src/web/studio/feed.ts) offers only Tell (a new tell to the app) or Steer; to comment on the issue or pull a post is about, a person must open it first.

Done when:

• [ ] A card whose item links to an issue or a pull (/:owner/:name/issues/:n or /pulls/:n) offers Reply beside Tell, for a viewer who may comment there; a reader who may not sees no Reply
• [ ] Reply posts through the same handler the issue/pull page's comment form uses (same-origin check, body limit, escaping all as there) and returns to the feed at that card with a flash, not to the issue page
• [ ] The comment shows up as a reply item in the feed (src/feed/items.ts already folds comments as replies)
• [ ] A test posts a reply from the home and proves the comment exists on the issue and the redirect returns to /; another proves a reader without comment rights is refused; both fail without the change
• [ ] Works with JavaScript off (plain form post)

Files: src/web/studio/feed.ts, the comment submit handler's return path, test/feed-reply.test.ts