[JENKINS-72947] Support symbols in TopLevelItemDescriptor#getIconClassName()

[mawinter69]

When adding a new Item via newJob the corresponding Descriptor can now make use of symbols

As a sample I modified the freestyle project to use a new symbol (svg source: https://www.svgrepo.com/svg/151600/package)

The image shows the old page layout with manual modifying the css in the browser

Alternate icon from @janfaracik with new layout

See JENKINS-72947.

Testing done

Manual testing opening newJob in root and in a folder
Added a new test that check svg is inserted and has proper class icon-xlg
Adjusted test for old iconClassName

Proposed changelog entries

• JENKINS-72947, support symbols in newJob page

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

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[janfaracik]

Worked on an icon for freestyle projects a while back - posting it here as an alternative:

https://github.com/janfaracik/jenkins/blob/c029ea4b4cfbea107fc904a4413613886f80090f/war/src/main/resources/images/symbols/freestyle-project.svg

[timja]

It would be good to send some PRs to other item types as well for consistency.

These are the ones I see in my test setup and would likely cover most of them:

[NotMyFault]

Thanks a lot!

[NotMyFault]

/label ready-for-merge

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This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!