Add welcome message when reaching wizard, navigate back to correct place when clicking Cancel

[mturley]

Resolves #105

Adds the following modal when you reach the import wizard page:

Note: at first we intended to have this modal only appear if you are not coming from the Add page, since the Add page card has a short similar description. To that end I implemented a ?from= URL parameter that I then reverted when we decided to have it show up no matter the source. If it ever becomes useful later, you can see that implementation in ea5f0c4 . This was also initially used as the method for navigating back to the right place on Cancel, before I realized we can just use history.goBack().

In order to implement the "don't show this again" checkbox, this PR also introduces a new common hook useLocalStorage that would be a good contribution to lib-ui later on (see migtools/lib-ui#106). This is a simpler alternative to the LocalStorageContext we implemented in forklift-ui.

[openshift-ci]

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[mturley]

This little refactor is an artifact of the reverted changes for the from URL param, but I left it this way because it's still an improvement (useRouteMatch is less awkward than using the RouteMatch prop type)

[openshift-ci]

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This pull-request has been approved by: ibolton336, mturley

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