Improve username logic and add navbar elements

[lucferbux]

Description

In this PR:

• Support username and isAdmin based on feat(bff): user info endpoint #630
• Add elements in navbar to mimic Kubeflow Central Dashboard
• Removes part of the TODOs we need to fix before the release

How Has This Been Tested?

1. In PoC mode and mock mode
2. Log in
3. Check that you can communicate with the backend

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages; the author will squash them after approval or in case of manual merges will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work.
• Code changes follow the kubeflow contribution guidelines.

If you have UI changes

• The developer has added tests or explained why testing cannot be added.
• Included any necessary screenshots or gifs if it was a UI change.
• Verify that UI/UX changes conform the UX guidelines for Kubeflow.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ederign

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• clients/ui/OWNERS [ederign]

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[ederign]

/lgtm