Updated the back button to be a proper link instead of a reference to
the last viewed URL since it could break if the last page was the
current one (On validation for example).
Includes test to cover.
Also applied some styleCI changes.
Fixes#2834
Done a little reorganisation while there of misplaced tests.
Moved MarkdownTest to a new PageEditorTest to avoid confusion with
other markdown elements and to align with other page tests.