This PR fixes #405 and provides some additional smaller enhancements intended to improve the safety and user-experience pertaining to creating Recovery Keys -- especially for ZKAPAuthorizer-enabled storage grids. Among them:

• On ZKAPAuthorizer-enabled grids, attempt to create the rootcap immediately once the first batch of ZKAPs becomes available for spending (rather than waiting until the voucher/batches are fully redeemed or until a user adds a folder) -- and fail loudly with a user-visible error dialog in the event that this fails.
• Prompt the user to create a Recovery Key after the rootcap has been created and if/when the "Folders" view is visible (i.e., before any folders have been added).
• Remind the user to create a Recovery Key once -- but only once -- per session in the event that one hasn't been created previously.
• Add a confirmation buttons ("Cancel", "Save Recovery Key...") to the PasswordDialog to give users clearer or more explicit instructions about how/whether to proceed. (Previously, continuing was triggered by an Enter/Return key press -- while cancelling was triggered by Esc -- which wasn't very discoverable.)
• Update user-facing language throughout the interface to remove the terms "export" and "import" in the context of handling Recovery Keys (as these were found to be confusing to some users); replace "export" with "create" (or "save" in the context of writing to disk), and "import" with "restore from".