Add experimental dashboard UI

[janfaracik]

This PR adds an experimental dashboard UI, hidden behind a flag, in line with the new job and run pages.

What's new?

• New design, consistent with jobs and builds
• Trying to reduce Jelly duplication (E.g. folder plugin duplicates stuff from core, would be good not to)
• App bar supports icons, so plugins don't need to implement that themselves
• Support for icons/badges for tabs

There'll be more to come/more polish in future iterations.

Testing done

• UI isn't ideal at the moment, but it is functional

Proposed changelog entries

• Add experimental dashboard UI

Proposed changelog category

/label web-ui,rfe

Proposed upgrade guidelines

N/A

Submitter checklist

• The Jira issue, if it exists, is well-described.
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Desired reviewers

@jenkinsci/sig-ux

Before the changes are marked as ready-for-merge:

Maintainer checklist

• There are at least two (2) approvals for the pull request and no outstanding requests for change.
• Conversations in the pull request are over, or it is explicit that a reviewer is not blocking the change.
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• If the change needs additional upgrade steps from users, the upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the pull request title (see example).
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[timja]

Tab bar needs to handle better many views:

I've seen many examples of places with 3-4 rows of views normally dashboards for teams.

[timja]

Branches is touching the table now when it wasn't before:

(guessing that branches will go away though?)

[janfaracik]

Tab bar needs to handle better many views:

I've seen many examples of places with 3-4 rows of views normally dashboards for teams.

Yup, I want to add an overflow in a similar way to the breadcrumbs.