Check for duplicate identifier

[rolfkleef]

As a user I want to receive a warning message when I enter an already existing activity identifier for a new activity.

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It's easy to accidently enter another activity with the same identifier, a warning that an identifier is already present can prevent duplicate entry or mistakes. For me there's no need to prevent a user from doing it, but two things will help a user detect and resolve this:

• a warning message upon saving such an activity
• perhaps a different color in the list of activities with a warning underneath "this identifier occurs multiple times"

[mesaugat]

This is the same issue as #100 and has been resolved now.