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.
Noah Petherbridge
885adda156