Enhancement: Epub reader app

We've added a epub reader app to open ebooks in web.
This includes a variety of functionalities such as a comprehensive chapter list, font size adjustment options,
and the ability to remember the user's last reading position even after the app is closed.

https://github.com/owncloud/web/pull/10448
https://github.com/owncloud/web/pull/10462
https://github.com/owncloud/web/pull/10468
https://github.com/owncloud/web/pull/10469
https://github.com/owncloud/web/pull/10470
https://github.com/owncloud/web/pull/10503
https://github.com/owncloud/web/issues/10464
