LG-3813: Extend session timeout on document capture step changes

[aduth]

Why: As a user, I expect that independent screens of document capture step ("Documents", "Selfie", etc) are treated the same as any other step in the identity proofing flow such that my session is extended when transitioning from one step to the next, so that I don't prematurely encounter a "Your session is about to expire" dialog if it takes me a while to complete the process.

Notes:

There was some technical discovery required by the ticket, addressed here:

• The dialog is shown when a result of a call to the /active endpoint returns a remaining_time less than the given threshold. Thus, it is enough to extend the session timeout to prevent the dialog from being shown in the same page session (source).
• Most interactions which incur a call to a server-side endpoint will extend the user's session. This includes asynchronous upload / logging, as well as the final submission to verify results. Thus, only the step changes (documents to selfie) were considered missing as far as behaviors which would be expected to extend the user session timeout.

[zachmargolis]

LGTM