Support User: Perform a complete page refresh when activating

[jordwest]

This performs a full page reload when a support user token is activated/deactivated to ensure all user data is fully refreshed. Between refreshes, the support token is stored in localstorage using the store module. On the following boot of Calypso, the User initialization sequence is intercepted before the original user is loaded, to ensure none of the original user's data is loaded.

Additionally, an error message is now displayed when an error occurs while fetching a token.

No other functionality has been changed; the testing procedure is the same (ref: p4TIVU-2OW-p2)

A few notes for review:

• The Redux store wasn't an option for storing the support token, as we need to check for the token at the earliest point during User module initialization, before the Redux store is initialized.
• Attempts to use localforage also caused issues due to its async API and the synchronous nature of the boot process - see Support User: Use localforage for storing the support token between Calypso restarts #3556

Fixes #3029, fixes #3067, fixes #3068 as well as various other non-reported issues (ref p4TIVU-2OW-p2)

cc @dllh @gwwar @mtias

[dllh]

I gave this a test drive, and it's working well for me functionally.

[gwwar]

Were the selector imports becoming too lengthy?

[jordwest]

Previously this selector was using the support user and token in the Redux store to determine whether support user was active. Now that we're storing those outside the Redux store, I've opted for just a boolean flag in the Redux state to keep track of whether support user is active or not. We could keep the selector, but it would just be:

const isSupportUser = ( state ) => state.support.isSupportUser

[gwwar]

👍 Nice work! ✨ It's really coming along.

One thing we may want to clear up in this PR or in an immediate follow up is that the currentUser id doesn't change back to the original user id after exiting the mode.

As is, I think it's good to 🚢 as an outstanding wip.

[jordwest]

Thanks for the review @gwwar. I've fixed up the typos and will create issues to track the other remaining bits.

For the time being I've fixed the issue with currentUser id not changing back to the original user by just nuking the store completely (as is done in User when it detects a change):

    rebootNormally() {
        debug( 'Rebooting Calypso normally' );

-       store.remove( STORAGE_KEY );
+       store.clear();
        window.location.reload();
    }

If there are no other objections I'll merge shortly

[gwwar]

As long as you're awake for the deploy, go for it. 😄