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See #90 for Problem with ExamType subscriptions

Fixed reverse query bug in Subscription model
Fixed bug in subscription view resulting in uncaught exception
Creating a subscription with a query/key/stage combination for which no assignments were available would result in an uncaught SubscriptionEnded exception and a 500 response to the client. Instead an error message with the status code 410_GONE is now sent.

Fixed reverse query bug in Subscription model

chnaged
type_query_mapper = {
	...
        SUBMISSION_TYPE_QUERY: 'type__title',
    }

to
type_query_mapper = {
        SUBMISSION_TYPE_QUERY: 'type__name',
    }

Refactored serializer id fields and camelCase names

To provide a uniform api and to save us from further work i've refactored the existing fields that used
camelCase names to use the names specified in the models (which are kebab-case).
Also everywhere where id's (whether normal or uuid ones) have been included in the serializers, the field names have been changed to 'pk' or '<model>_pk'. Pk will always link to the primary key of the model and will save us great pain should we decide to convert the pk's of more models to uuid's. Also we won't have to remebre a bunch of different ways of referring to the id for the frontend, it's always pk.
I also included the pk field in all modelserializers since this will be necessary for the frontend state management.

Frontend now expects pk fields and snake_case

Solution is highlighted / Desc. HTML is rendered

Frontend test is only manually run

Added vue-notification library

Inactivity detection preperly implementd

Client inactivity is now properly detected. A vuex plugin is used to store the time of the last commited mutation. This roughly equals the last user interaction.
If the users session is expired he will be redirected to the login page. Before that a dialog is displayed notifieng the user that they are about to be logged out.

Added created / of_tutor info to feedback comment