Unify BookEditView and AddBookWizard step 2 views

[sinergatis]

Make BookEditView and AddBookWizard share the template and the forms (by making BookMetadataForm a subclass of BookEditForm), plus cleanup the templates, forms, and related stuff in the process. This should make the code more maintainable and allow for further changes in the edit/import functionality easier to keep in sync, as discussed at #39.

The changes on books/epub.py (and in turn commands/addepub.py) revert some previous changes that were not handled properly: when modifying models or epub.py, it's important to make sure it does not break things elsewhere - as we do not have proper tests for the UI yet, please do be careful and check manually when introducing such changes!

Things that would need work in further iterations:

• tags are currently not imported when uploading a book. This is mainly related to the books/epub.py comment - we probably want to move the "cleaning up" of the authors/publishers/tags/etc (strip, check for empty, cleanup) there, in order to avoid having to clean them up both on addepub and on the wizard.
• prettifying the templates.
• run makemessages and adjust where necessary (and double check for untranslated strings)

[sinergatis]

Merged - as usual, please review whenever you have the chance and follow up if needed (in particular due to the lack of makemessages and possible dancing around of the translation strings).