Fix bug 9686: Slowdown after heavy usage
bug 9686: avoid spelling error
bug 9686: avoid ever-growing @ISA in CustomerCompany and CustomerUser
use "first" instead of "grep"

as suggested by reneeb
remove stray conflict marker and module import
"use base" instead of pushing to @ISA
CHANGES.md entry for bug #9686
reset @ISA before pushing to it
encourage sane coding practices in the documentation
Merge branch 'bug-9686' of https://github.com/moritz/otrs