Restore "Enable" button

[daniel-beck]

#419 removed the "Enable button" without a justification: Core did not remove that button from Freestyle projects in jenkinsci/jenkins#9287, so aligning with core behavior shouldn't remove it either.

Freestyle for comparison:

Folder before (after #433):

Folder after:

Manually tested by clicking the button on an empty MBPL in hpi:run and it worked.

Proposed changelog entries

• Allow enabling certain folders, like Multibranch Pipelines, from their main page again, if they're disabled.

Submitter checklist

• JIRA issue is well described
• Changelog entry appropriate for the audience affected by the change (users or developer, depending on the change).
  * Use the Internal: prefix if the change has no user-visible impact (API, test frameworks, etc.)
• Appropriate autotests or explanation to why this change has no tests