Fix issues with pdf viewer

[joandste]

Closes #633, Closes #634

[keanderss]

Currently when viewing a document that has an even page count the page ordering isn't retained when going backwards and the second page won't be displayed at all. For documents with an odd page count everything works as intended. The changes make it so that this behaviour is now flipped and will break documents with an odd page count in the same way while fixing it for documents with an even page count.

[joandste]

Yeah sorry, i just assumed we were going to always use even pages, because then it will always be possible to show the front and backcover as a single page

Im guessing we want to show a double view on the last page if document has an odd number of pages?

Though books cant have an odd number of pages.

[keanderss]

For the current use case of the publications app it won't be a problem as it is only used for UpDaTe but this could change in the future.