Add a feature where recent public channel messages can be echoed back to
newly joining users when they enter the chat room.
* Configure in settings.toml with EchoMessagesOnJoin. 0 = disable storage.
* Messages are stored in RAM and lost on a server reboot.
* A buffer of recent public messages per channel can be kept, e.g. for the
10 most recent messages.
* The settings can be reloaded with /reconfigure and the message buffers
will rebalance on the next message sent.
* When a new user logs in, a new "echo" message is sent that contains all
of the echoed messages on a "messages" list, in one WebSocket packet.
* Echoed messages are put above the ChatServer welcome messages.
* If a message is taken back, it's removed from the echo message buffer.
Other Changes
* Don't broadcast Presence messages within 30 seconds of the server boot, to
lessen a flood of messages when a lot of users were connected at reboot.
* Change the default "Join messages" setting on front-end to hide them in
public channels.
* For the admin buttons in ProfileModal, use the AlertModal instead of native
browser prompts.
* On the /nsfw command, BareRTC will issue an admin report to your main website
so you have visibility into when this command is used.
* On the server side, fix the "Open" command so it will prevent webcams from
connecting if the offerer had been Booted by the answerer, in addition to the
previous blocks about Mute and Block. Admin users can still connect always.
* Add a debug command `/watch username` to manually open somebody's camera on
chat. Note: the chat server enforces the permission to actually do so.
* Remove the /debug-dangerous-force-deadlock admin command.
