Replace room_invite_state_types with room_prejoin_state (#9700)

`room_invite_state_types` was inconvenient as a configuration setting, because
anyone that ever set it would not receive any new types that were added to the
defaults. Here, we deprecate the old setting, and replace it with a couple of
new settings under `room_prejoin_state`.

docs/code_style.md

@@ -128,6 +128,9 @@ Some guidelines follow:
     will be if no sub-options are enabled).
 -   Lines should be wrapped at 80 characters.
 -   Use two-space indents.
+-   `true` and `false` are spelt thus (as opposed to `True`, etc.)
+-   Use single quotes (`'`) rather than double-quotes (`"`) or backticks
+    (`` ` ``) to refer to configuration options.
 
 Example: