Remove global "auto_join_rooms" flag and use optional "/join" segments in invite links

Generated for channels, not for "normal" rooms.

src/components/welcome_headers/WelcomeHeaderChannelUser.vue

@@ -1,6 +1,6 @@
 <template>
   <div class="created-room-welcome-header">
-    <div v-if="$config.auto_join_rooms" style="white-space: pre-line;">
+    <div style="white-space: pre-line;">
     {{ $t('room_welcome.info_auto_join',{room: room ? room.name : "",you: $matrix.currentUserDisplayName}) }}
       <a href="#" text @click.prevent="viewProfile">
         {{ $t("room_welcome.change") }}