Custom homepage usage will now be checked before any actioning
of deletion rather than potentially causing an exception acting
during the deletion.
Previously a deletion could still be created, within the recycle bin,
for the parent which may lead to the page being deleted anyway.
For #3150
Removed some common functions from other entities.
Aligned implementation of getUrl()
Cleaned phpdocs and added typehinting.
Also extracted sibling search logic out of controller.
Tools seems to fit better since the classes were a bit of a mixed bunch
and did not always manage.
Also simplified the structure of the SlugGenerator class.
Also focused EntityContext on shelves and simplified to use session
helper.
