Text changes for global and programme fields

Now using 'immutable' of global fields and 'overridable' for the programme-level overrides
Fixed bug with undefined index in simpledata.php replace_ids_with_values()

When no value is set in a drop down, the option is given an index 0, which causes an error
Merge branch 'develop' of github.com:unikent/programmes-plant into feature/add-preview-api-outputs
Handle empty secondary subject.