This commit includes a new `stream_post_policy` setting,
by replacing the `is_announcement_only` field from the Stream model,
which is done by mirroring the structure of the existing
`create_stream_policy`.
It includes the necessary schema and database migrations to migrate
the is_announcement_only boolean field to stream_post_policy,
a smallPositiveInteger field similar to many other settings.
This change is done to allow organization administrators to restrict
new members from creating and posting to a stream. However, this does
not affect admins who are new members.
With many tweaks by tabbott to documentation under /help, etc.
Fixes#13616.
This experimental setting disables sending private messages in Zulip
in a crude way (i.e. users get an error when they try to send one).
It makes no effort to adjust the UI to avoid advertising the idea of
sending private messages.
Fixes#6617.
We're not sure this feature is the best solution to this category of
problem, in that use of this feature might cause spam to stick around
longer, vs features that encourage immediate deletion.
Tim Abbott
Ryan Rehman
Rishi Gupta
David Wood