Make scrolling up in chat panel smoother

This increases the threshold at which we start loading new messages
as well as the amount of messages we get back from the server every
time we fetch.

Also, we restructured the seed binary to use the methods in `Db` to
generate seed data and added random chat messages.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

script/seed-db

@@ -2,4 +2,4 @@
 
 set -e
 cd server
-cargo run --bin seed
+cargo run --features seed-dependencies --bin seed