Admin UI blank state banner

Review Request #10873 — Created Feb. 1, 2020 and updated — Latest diff uploaded

Review Board
10979, 10978

This RB creates the new component for improving usability of the admin
page. Currently, it's not very clear what new users need to do when they
come to the admin UI for the first time.

This banner displays the various steps that an admin needs to complete
in order to start using their Review Board instance with links to the
corresponding setting and documentation. The banner can be dismissed
using the 'x' in the top right corner and the user can view different
slides by clicking on the corresponding pagination dot or using the
'previous' and 'next' arrows.

The pagination dots also show the current state of the instructions
in the slide. Once a slide is viewed, if it's an optional slide, the
corresponding dot is filled in with an info icon. If it's a slide that
requires verification, the dot will be left empty until the step is
completed. Once the step is completed, the dot becomes checked. For the
slide currently being viewed, the dot is solid or filled in.

When user navigates to another page after having interacted with the
banner, the state of the banner is preserved. That is, the banner will
show the slide that the user left on and whether other slides were
viewed. The state of the banner is reset if the banner is dismissed.

Finally, users can toggle showing or hiding the banner by going to the
Support setting in the administration page.

Unit test coverage:
- rendering the welcome slide
- rendering a slide with content
- rendering the paginator
- the showSlides method with an index within the slide range, above the
slide range, and below the slide range
- previous, next, pagination dot, and dismiss buttons

Manual testing:
- using the prev and next buttons to navigate between slides
- clicking the dismiss button hides the banner
- clicking a pagination dot changes to the correct slide
- links for documentation and to the settings are correct

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