dbhub: Improve UI for pushing databases

This improves the user experience for pushing database files. Before
this you had to type in the entire URL for pushing, e.g.:
https://db4s-beta.dbhub.io:5550/username/databasename
With this the host name as well as the user name is taken from the
currently active client certificate. So all you have to type in now is
the database name. And for this we make a sensible automatic suggestion
based on the name of the local file you're trying to push.

Note that while this makes pushing databases a lot easier, it still
doesn't implement proper version control or any extra code for handling
conflicts etc.

src/RemoteDatabase.h

@@ -22,8 +22,15 @@ public:
 
     void reloadSettings();
 
+    enum CertInfo
+    {
+        CertInfoUser,
+        CertInfoServer,
+    };
+
     const QList<QSslCertificate>& caCertificates() const;
     const QMap<QString, QSslCertificate>& clientCertificates() const { return m_clientCertFiles; }
+    QString getInfoFromClientCert(const QString& cert, CertInfo info) const;
 
     enum RequestType
     {