Improve and simplify SCSS classes and variables

[janfaracik]

This is a small PR to improve and simplify some classes and variables used in Jenkins.

• The small border animation on clicked/focused elements is now a reusable mixin, which cuts down on code duplication and brings greater consistency to the newer Jenkins components
• The item() mixin (as described above, happy to hear suggestions for a better name) now uses variables for its background/shadow meaning that it's more theme-able than before
• Manage Jenkins SCSS has moved to the /pages folder
• Minor font size changes to the Manage Jenkins to bring more balance to the page + make it more responsive
• The plugin update bubble now pops in on page load for increased visibility

Screen.Recording.2022-06-16.at.17.46.07.mov

• Some very minor color changes to increase vibrancy
• A new keyboard focus state for components using the item() mixin

Proposed changelog entries

• N/A (alternatively Minor color changes to UI components)

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• 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
• Appropriate autotests or explanation to why this change has no tests
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadoc, as appropriate.
• For dependency updates: links to external changelogs and, if possible, full diffs

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).

[timja]

LGTM tested via gitpod and compared it with https://weekly.ci.jenkins.io

[NotMyFault]

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!