Fix for DOCS-81: Evaluate impact of a move to docbkx-tools 2.0.15

There are some changes affecting the layout of HTML, and the fact that EPUB files now seem to be produced one level up.

There are also some changes due to how 2.0.15 ignores <targetsFilename>.

Furthermore, RTF generation fails with 2.0.15, so these changes for 2.0.14 for RTF.

Rather than favor backwards compatibility, this change instead moves the plugin forward to take advantage of the changes in 1.78.1 stylesheets.