Move the GET /xxx/{id}/categories requests to the corresponding classes
(Contacts, Products, Thirdparties).
Move the addCustomerCategory() method to the Thirdparties class. Use the
POST verb for the request instead of GET because the request modifies
the state of the server.
Remove several /thirdparty/list/xxx endpoints because they are redundant
with the /thirdparties/ endpoint whith the 'mode' filter.
Remove the customer/byemail/{email} endpoint because it is redundant
with the /thirdparties/ endpoint whith the 'email' filter.
Remove all the /customer/ endpoints because they do the same thing as
the corresponding /thirdparty/ endpoints.
