Create posts from chat

Start a new post from any chat message to move the conversation into a more permanent, async-friendly space. Everyone in the chat thread will be notified that a post was created as a reminder to move the conversation.

Other improvements

• Added ↑/↓/j/k navigation through the posts index page. When viewing a post, you can also press j/k to navigate to the previous/next post without ping-ponging back to the previous page.

Fixes

• Fixed a bug that prevented the new call button from appearing on newly-created chat threads.
• Fixed formatting for text with multiple combinations of bold, italics, underlines, strikethroughs, and links applied at the same time.
• Prevents keyboard shortcuts from repeating when keys are held down by default.
• Improved the follow-up hint date on the posts index page when a follow-up is in the far future.
• Fixed a case where the post title could display twice on compact posts without recent activity.
• Fixed an incorrect system message string when removing a group chat thread name.
• Improved error handling while generating previews for attachments in the post composer.