Remove uses of Quotient:Omittable

Note this technically won't build for now because of the lack of RoomMember support but I'll push that at the quotient-next branch next. 

This is needed as well to get a branch that builds on dev.

src/jobs/neochatchangepasswordjob.cpp

@@ -5,7 +5,7 @@
 
 using namespace Quotient;
 
-NeochatChangePasswordJob::NeochatChangePasswordJob(const QString &newPassword, bool logoutDevices, const Omittable<QJsonObject> &auth)
+NeochatChangePasswordJob::NeochatChangePasswordJob(const QString &newPassword, bool logoutDevices, const std::optional<QJsonObject> &auth)
     : BaseJob(HttpVerb::Post, QStringLiteral("ChangePasswordJob"), "/_matrix/client/r0/account/password")
 {
     QJsonObject _data;