Miscellaneous modernization cleanup

[NotMyFault]

This PR polishes several components:

• Utilize the computer symbol in SlaveComputer
• Use the cogwheel symbol to edit an agent, like previously.
• Adapt the color scheme on drop downs
• The text preview box is now less sharp
• Single line -> multi line expanders now expand into a jenkins-input to benefit from the modernization
• Other minor changes

Proposed changelog entries

• Miscellaneous polishing of various components.

Proposed upgrade guidelines

N/A

Submitter checklist

• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change) and are in the imperative mood. Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade

Desired reviewers

@jenkinsci/sig-ux

Maintainer checklist

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

• There are at least 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 does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[alecharp]

I tested using a docker jenkins/core-pr-tester container. It seems to be fine, expect that some localization are not respected. I tested using the Locale plugin. I'm only mentioning this for global security configuration side panel addition.

[timja]

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!

[NotMyFault]

Is there anything that blocks a merge @timja?

[timja]

Is there anything that blocks a merge @timja?

No I slipped up on not adding the label, no real rush though generally we just make sure the PRs are merged by Tuesday to make the weekly, the 24 hours is more a minimum than an actual time.