8.0 : Add module hr_expense_analytic_default

This PR is the equivalent for v8 of the following PR for v7: https://github.com/OCA/hr/pull/44
Please comment on the PR for v7 (unless your remark is specific for v8)
I don't want to put it to invisible, because that way the user can change the default analytic account for a particular expense note, which is quite usefull. For example, a guy from R&D (who has a default analytic account "R&D" on the employee form) is sent by its company to an exhibition and the budget of the exhibition is on the Marketing budget. So, when this employee will create its expense note for that exhibition, he would set the "Analytic account" on hr.expense.expense to "Marketing" and then create the expense lines.