Bug 3522 Refactor Create Update Delete letter for consistency and readability

Bug 3522 Refactor Create Update Delete letter for consistency and readability

Fixed inconsistent usuage of primary key in delete
Refactored:
Moved operations to separate subroutines to clarify data flow
Removed unnecessary redirects to self
Renamed confusingly named else variable passed to template
Other changes for code clarity
NB Outstanding:
database reading/writing should live in appropriate module
not duplicate it here.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>