[FIX] hr_expense: Solve several usability issues on the flow

Review the subtypes
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- Add a subtype for the paid expenses
- Correct subtypes wording

Prevent expense report for several employees
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- Can add lines from every employee, leading to account issues (payable / receivable)
  Raise an error if you try to report several expenses for different employees

Don't track onchanges on journal_id
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- Expense Sheet, click on Post Journal Entries: you have 2 produced messages, and
  one is strange (Journal Entry: * 1 ?)

Remove useless message on chatter
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- Remove the message to say that the expense report has been validated by ...
  It's redundant as the visibility tracker already logs this change

Remove unused method
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- 'refuse_expense_sheets' is declared but never used

Log a link to the payment on chatter
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Expense Sheet, click on Register Payment: you have a message containing the
payment name (SUPP.OUT/2016/0001). Add a link to the form view.

Expense duplication
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Do not copy 'sheet_id' on duplication. It's wrong and has as secondary effect
to duplicate the state too.

Expense Report tree view
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Display correctly the widget monetary

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