Let tours be skipped on view enter and started later imperatively

[pkalita-lbl]

Fixes #231

The StudyView component conditionally launches on modal on data fetch. If it (or any components that it renders) starts a tour on view enter and the modal is opened soon after when the data fetch settles, the result is a tour highlighting an element that is hidden under the modal.

These changes address this by:

• Adding a prop to the useAppTour hook that allows a client to turn off initiating the tour on view enter
• Adding a return value from useAppTour that includes a function which can imperatively start the tour (or at least attempt to, depending on whether the tour has already been presented)
• Use an extra bit of state in StudyView to control whether it is safe to present tours. StudyView and its children can observe this state to imperatively start a tour when it's safe to do so.

[eecavanna]

LGTM! Thanks for addressing this! I left one comment about boolean prop/variable names. I don't consider it a deal-breaker—just my thoughts as I was reading the code. I'm OK with the branch being merged in as is.

[eecavanna]

Not a deal-breaker, from my perspective:

Regarding variable names: when I see the names tourAllowed/toursAllowed, I think they may contain identifiers of tours that are allowed, as opposed to boolean flags. For that reason, I think I'd prefer prop/variable names that have is/are in them (e.g. isTourAllowed, areToursAllowed).

I don't want to hold up a merge for this. With the TypeScript type information and React.useState default value that are present, I (someone reading the code) can resolve the ambiguity.