GUI/BLOCKSERVER: Changing the synoptic of a configuration after deletion of an in use synoptic.

Currently, when an in use synoptic is deleted, the configurations using the synoptic display a peculiar set of behaviours:

- If the current configuration's synoptic is deleted then it will display "-- NONE --" as its new default synoptic. However, if an EPICS command is used to ask what synoptic is being used in the configuration, it will return the synoptic that has just been deleted, and the configuration.xml file will display the same information.
- If a non-current configuration's synoptic is deleted then it will display the current configuration's synoptic as its new default synoptic, although, once again, the changes are just on the gui and neither displayed by the EPICS command window or the xml file.
- On load, the synoptic is displayed but not set to " --None-- ".

As a user, I would like the configuration to display "-- NONE --" as its synoptic, after deletion of it's previously used synoptic.

Acceptance criteria:
- On edit or load of configuration whose synoptic does not exist the NONE is selected.

This issue originated here: https://github.com/ISISComputingGroup/IBEX/issues/3211