Support User: Add user interface components

[jordwest]

This adds a keyboard shortcut and login box for the support user feature. Props to @mrheston and @jmdodd for the original work on this.

Ref p195om-2GO-p2

cc @dllh

[mtias]

This could be { config.isEnabled( 'support-user' ) && <SupportUser /> }

[jordwest]

Done :)

[dllh]

I tested this a little, and it works well for me. As noted elsewhere, I do see this js error when I use the feature: "Warning: Failed propType: Invalid prop user of type array supplied to Gravatar, expected object. Check the render method of MeSidebarNavigation."

I think that's not related to this PR and that it's related to the issue reported in #3029.

[jordwest]

Thanks for the feedback. I've made a few changes in response.
@dllh: I'm now tracking that issue in #3066

[dllh]

@mtias mind giving this another look to confirm that your suggestions are adequately addressed?

[dllh]

I took another look at the code, and it looks good to me and seems that the requested changes have been made. I also gave it another test drive, and other than some known issues recorded elsewhere, it works well. Two pieces of feedback that I think we should jot down elsewhere rather than trying to piggyback on this PR:

• Can we focus the form field on its invocation?
• Should the masterbar de-highlight be delayed until state is returned to normal?

Let's 🚢.

[mtias]

The new directory placement looks good to me.

[jordwest]

Can we focus the form field on its invocation?

I've opened #3201 for this

Should the masterbar de-highlight be delayed until state is returned to normal?

Currently the highlight changes at the moment the support token injection changes - meaning any API calls from that point will be injected/not injected with the token. I think delaying the change could open us up to instances where the user falsely believes they are acting without/without the support token being injected. What we could do is to place an overlay across the whole page, preventing interaction until all data has been refreshed.