Hide own joins for direct rooms

Move display options to a new "roomDisplayOptionsMixin". Rename (and invert) "showOnlyUserStatusMessages" to "showAllStatusMessages".

src/components/roomDisplayOptionsMixin.js

@@ -0,0 +1,18 @@
+export default {
+  computed: {
+    showOwnJoins() {
+      return !this.$matrix.isDirectRoom(this.room);
+    },
+
+    showAllStatusMessages() {
+      // We say that if you can redact events, you are ad admin.
+      // NOTE!!! This assumes that there is a property named "room" on THIS.
+      const me = this.room && this.room.getMember(this.$matrix.currentUserId);
+      let isModerator =
+        me && this.room.currentState && this.room.currentState.hasSufficientPowerLevelFor("redact", me.powerLevel);
+    const show =  this.$config.show_status_messages;
+    return show !== "never" && (show !== "moderators" || isModerator)
+    },
+  },
+}
+