Unify the way we decide whether a room is a DM or a group room (#3100)

* Add centralised 'room is DM' check

Also add extension functions for `MatrixRoom` and `MatrixRoomInfo`.

* Use the centralised method and extension functions through the app, including:

- Room list.
- Room details screen.
- Invites.
- Notifications.

Replace most `isDirect` usages with `isDm`.

* Update screenshots

---------

Co-authored-by: ElementBot <benoitm+elementbot@element.io>

features/invite/impl/src/main/kotlin/io/element/android/features/invite/impl/response/AcceptDeclineInviteView.kt

@@ -79,13 +79,13 @@ private fun DeclineConfirmationDialog(
     onDismissClick: () -> Unit,
     modifier: Modifier = Modifier
 ) {
-    val contentResource = if (invite.isDirect) {
+    val contentResource = if (invite.isDm) {
         R.string.screen_invites_decline_direct_chat_message
     } else {
         R.string.screen_invites_decline_chat_message
     }
 
-    val titleResource = if (invite.isDirect) {
+    val titleResource = if (invite.isDm) {
         R.string.screen_invites_decline_direct_chat_title
     } else {
         R.string.screen_invites_decline_chat_title