Implement fetching software system vendors from API for pull-down menu in CreationPage

[Torgeir333]

Issue #67 has now provided a new GET endpoint,
described both in Swagger, and in issue #8,
so that the pull-down menu in CreationPage,
showing software vendor systems
available for a new SystemUser.

Thus we will implement a Redux fetch function to GET the data,
and then populate the pull-down menu with the data from Redux State.

Tasks

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1. Set up a new Redux fetch function in the CreationSlice
2. Set up asynchronous listener (ExtraReducer) in CreationSlice to populate Redux State
3. test new creationPage Redux State using Chrome DevTools
4. Use the new fetch function once when OverviewPage first renders
5. Populate pull-down menu with data from Redux State rather than local mock data used so far