Update TaskTree design

[AlanGreene]

Extracted from #1445 (comment)

Update the design of the TaskTree component to:

• reduce use of heavy block colours
• make it easier to see status of multiple TaskRuns / steps
• etc.

Here's a rough mockup of the proposed design:

There are still some details to be worked out but I think this would be a nice improvement.

[AlanGreene]

/kind design

[AlanGreene]

StatusIcon component has been updated and provides 2 styles: default and 'inverse'.
The inverse style currently uses a white fill and is intended for use on the block colour backgrounds.
With the new design this can switch to the outline style instead. May need a minor update to remove the exclusions.

[AlanGreene]

/assign

[AlanGreene]

I have the initial updates mostly complete but will hold off on the PR until after the next Dashboard release to avoid holding up getting some important fixes out (e.g. rerunning cancelled runs #1788)

[AlanGreene]

Some focus states still need to be updated, e.g. selected step, but otherwise it's nearly there.

I've made some changes to StatusIcon size and positioning.

This first pass maintains the existing behaviour of only having a single TaskRun expanded at a time. A future update will add the chevron to allow multiple to be expanded at the same.

[AlanGreene]

PR #1806 with the initial design update is merged, will check it out on robocat after the next nightly deploy.

The next step is to support expanding multiple TaskRuns at the same time so the user can see summary of step status without having to switch between them.