[REF] hr_expense: use new implementation for auto subscription

Expense report has custom code to add people as followers. This code has
several issues, notably in some cases it could try to add several times
same people, leading to crash and issues.

As auto subscription has been cleaned let us clean the code in hr expense.
Using _message_auto_subscribe_followers it is now simpler to automatically
add follower to expense reports. It is not necessary to add hackish custom
code in create and write.

One change is done in this commit. Some people are set as follower after
record creation and not before. However this was necessary when tracking
messages were not correctly triggered or auto subscription done after
tracking. As process is now 1/ create 2/ auto subscribe 3/ tracking, messages
linked to tracking will be correctly push to recipients automatically added
as followers.