Invite users to existing rooms (#441)

Invite users to existing rooms

Scope:

- Allow inviting from the room detail screen and the member list
- Invite option is only shown if the user has the correct power level
- Search flow the same as creating a new room, allowing multi-select
- Existing room members/invitees are disabled with a custom caption
- Sending is asynchronous, an error dialog will appear wherever the
  user is if necessary

Closes #245

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/room/MatrixRoom.kt

@@ -85,4 +85,8 @@ interface MatrixRoom : Closeable {
     suspend fun acceptInvitation(): Result<Unit>
 
     suspend fun rejectInvitation(): Result<Unit>
+
+    suspend fun inviteUserById(id: UserId): Result<Unit>
+
+    suspend fun canInvite(): Result<Boolean>
 }