* src/IDF/Key.php: new column "type" which is either "ssh" or "mtn";
utility functions to query the mtn key name and id as well as
all available key types for the current IDF installation
* src/IDF/Migrations/16KeyType.php: needed migration script
* src/IDF/Plugin/SyncGit/Cron.php: ensure only SSH keys are handled
* adapt forms and templates accordingly