Enable motd on non-pocket and command computers

- I'm excluding pocket computers, as they have such a tiny screen I'm
   not sure the screen estate is worth it.
   Pocket computers /generally/ aren't people's starter machine, so I
   think this is fine.
 - Prune the motd list, and try to make them a little shorter. I think
   this list is more of the interesting ones. We can modify this list in
   the future, as we get more feedback.[^1]
 - Also fix paint/edit not adding an extension when they should. This
   was caused by the settings rewrite, as the explicitly provided
   default shadowed the one provided by bios.lua.

[^1]: ~5 months ago I asked for some feedback about enabling motds by
      default. I only got something constructive back today >_>.

src/main/resources/assets/computercraft/lua/bios.lua

@@ -923,7 +923,7 @@ settings.define("list.show_hidden", {
 })
 
 settings.define("motd.enable", {
-    default = false,
+    default = pocket == nil,
     description = "Display a random message when the computer starts up.",
     type = "boolean",
 })