This cleans up the reply_warning feature in favor of a more coherent
explanation of whether or not one can reply.
(Also, critically, it now advertises the ability to enable
missed-message email replies with some administrative configuration
work.)
This renames references to user avatars, bot avatars, or organization
icons to profile pictures. The string in the UI are updated,
in addition to the help files, comments, and documentation. Actual
variable/function names, changelog entries, routes, and s3 buckets are
left as-is in order to avoid introducing bugs.
Fixes#11824.