feat: big update for manage contributors page

[thesuperzapper]

This PR resolves a lot of outstanding issues with the central dashboard's "manage contributors" page.

The highlights are:

1. Allow users who are owners of multiple profiles to manage contributors of each:
   • closes feat: allow profile owners to select namespace under "manage contributors" #7435
   • resolves Dashboard's contributors section doesn't show all the namespaces I own #4569
   • resolves Cannot select which profile to manage in "Manage Contributors" UI #4924
   • resolves Manage Contributors UI Does not show all owned namespaces #6575
2. Display all profiles that users are a member of, split up by role:
   • Owner
   • Contributor
   • Viewer
3. Significant style improvements (see image below)

Screenshots

Normal User

This screenshot shows a normal user who is not an owner of any profiles, but is a "Contributor" 1 profile and "Viewer" of another.

Admin User

This screenshot shows a user who is an owner of multiple profiles:

Cluster Admin

This screenshot shows a "cluster-admin" user (who can manage all namespaces):

[juliusvonkohout]

We probably have to harden the viewer cluster role later on to make escalations from viewer to editor at least more difficult, but the user interface is so much better this way.

Merging this soon and later iterative improvements is what i would go for.

/lgtm

[ederign]

The UI is much better, thank you so much for this @thesuperzapper

/lgtm

[thesuperzapper]

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: thesuperzapper

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• components/centraldashboard/OWNERS [thesuperzapper]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment