Ensure links in help files open in new windows

[daniel-beck]

It's annoying when links in HTML help files that get shown inline after clicking "❔" are not opening in a new window: The current form may have modifications we want to keep.

Rather than fix every plugin ever, this PR changes hudson-behavior to rewrite the inlined HTML to ensure all links open in a new window unless they explicitly already specify the target attribute. (It can be set to _self explicitly to not open in a new window.)

Additionally, some cleanup: _new isn't a thing (😭), it just behaves similar to _blank because there's no frame (or popup? It's been a while…) with that name already. In help files, remove target entirely and rely on the JS added elsewhere, in HTML shown inline directly on config forms, open in a new window.

Proposed changelog entries

• Ensure that links from inline help (shown after clicking "?" icons) always open in new windows.

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). 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
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

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 correct
• 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]

not tested but looks sensible

[jglick]

Nice trick.

[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.