Update string literals

Since _ls is now deprecated this is removed in favour of L1, I've also taken the oportunity to replace QStringLiteral and QLatin1String with their shortened form while we're at it.

There are also a few instances where the string literal type has been switch, the general rule being to use the one that matches the function type or value being compared to avoid conversions.

src/models/userlistmodel.cpp

@@ -104,7 +104,7 @@ QVariant UserListModel::data(const QModelIndex &index, int role) const
         // User might not in the room yet, in this case pl can be nullptr.
         // e.g. When invited but user not accepted or denied the invitation.
         if (!pl) {
-            return QStringLiteral("Not Available");
+            return u"Not Available"_s;
         }
 
         auto userPl = pl->powerLevelForUser(memberId);