Modernise the plugin UI

[janfaracik]

This PR intends to update the plugin's interface, bringing it inline with newer pages in Jenkins.

Before

After

Before

After

Changes

• Use the app bar
• Drop the side panel
• Use a Jenkins Symbol rather than a PNG for the action icon
• Delete action.jelly and move its permission check inside of Action.java (the new header doesn't use action.jelly therefore the permission check to show the action wasn't applied)

Testing done

Submitter checklist

• [ ] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ ] Ensure that the pull request title represents the desired changelog entry
• [ ] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

java.lang.AssertionError: privileged should be able to see the Support action expected:<1> but was:<0>
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.failNotEquals(Assert.java:835)
	at org.junit.Assert.assertEquals(Assert.java:647)
	at com.cloudbees.jenkins.support.SupportTestUtils.testPermissionToSeeAction(SupportTestUtils.java:282)
	at com.cloudbees.jenkins.support.actions.SupportAbstractItemActionTest.onlyAdminCanSeeAction(SupportAbstractItemActionTest.java:46)

[judithlr]

For the support agent icon we can use this one. Not the best either but probably better than the other one. https://fonts.google.com/icons?selected=Material+Symbols+Outlined:support_agent:FILL@0;wght@400;GRAD@0;opsz@24&icon.query=agent&icon.size=24&icon.color=%231f1f1f

[lemeurherveCB]

Another suggestion a bit more legible for the icon: https://fonts.google.com/icons?icon.query=lifebuoy&icon.size=24&icon.color=%231f1f1f