consider memoizing dropdown lists for faster loading when another is requested

[cjyetman]

Currently, clicking "+ Add *" to add another dropdown selector triggers a new load of data from Microsoft Graph API, which can cause a perceivable delay in making the new dropdown useable. Since the available items in any dropdown are unlikely to change during a single user session, it might be advantageous to memoize the dropdown lists so that a previously retrieved dropdown list can be used if nothing else has changed, e.g. using React's memo.