Add Element Call participant limit (#23431)

config.sample.json

@@ -46,6 +46,7 @@
     },
     "element_call": {
         "url": "https://call.element.io",
+        "participant_limit": 8,
         "brand": "Element Call"
     },
     "map_style_url": "https://api.maptiler.com/maps/streets/style.json?key=fU3vlMsMn4Jb6dnEIFsx"

docs/config.md

@@ -323,6 +323,8 @@ The VoIP and Jitsi options are:
       and may be removed at any time without notice. Defaults to `https://call.element.io`.
    - `use_exclusively`: A boolean specifying whether Element Call should be used exclusively as the only VoIP stack in
       the app, removing the ability to start legacy 1:1 calls or Jitsi calls. Defaults to `false`.
+   - `participant_limit`: The maximum number of users who can join a call; if
+     this number is exceeded, the user will not be able to join a given call.
    - `brand`: Optional name for the app. Defaults to `Element Call`. This is
      used throughout the application in various strings/locations.