Add download option to 'Console output', move 'View as plain text' and 'Copy' buttons to app bar

[janfaracik]

Companion to jenkinsci/jenkins#9169. Adds a download option to 'Console output', moves 'View as plain text' and 'Copy' buttons to the app bar. Requires 9169 as a base to show the actions in the app bar, otherwise doesn't show on lower versions of Jenkins - nothing breaks.

Would be good to have the console page be extensible and have this plugin replace just the parts it needs - but out of scope for this PR.

Testing done

• Copy button works as before
• View as plain text works as before
• Download button downloads the full log

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