Modified mail notifications to add the name of the book

[Man-in-Black]

As written on discord I've changed the mail notifications to add the title of the book.
It's my first real pull request, so I hope I've done everything right ;)
This time with the right codebase ;)

[Man-in-Black]

I've just got the idea to extend the whole notification, so as fpr now it is showing the book name as well as the chapter name if present.

without chapter:

with chapter:

[ssddanbrown]

Thanks again for this @Man-in-Black.
I've now merged this in via commit 65ac197 (I squashed your changes down to a single commit to clean up the history of changes).

I followed this up with d41fd7a, which added testing and tweaked the format a bit so that both the parent chapter and book are instead shown as a "Page Path:" item in the details list, with the names linked to the item in BookStack. This is to keep the details focused on pages being the main context of the email.
The changes in this commit also add testing, permission control and add these details also to the comment notification.

This will be part of the next feature release.