Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:
## 

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1/ Modify the no content help message for hr_expense with a dynamic part
   which describe how to create a new expense by mail
2/ Mail gateway mechanism. New customizable email alias expense@domain
   that creates a new expense by sending an email to it. Check several things
- Check that the email_from is the same than one of the employees or than
   on of the related users to employees. If not, send back an email to say that
   the expense will not be created.
- If the email address is valid, check if something is between brackets '[]'
   If it is the case, check if it is linked to a product internal reference
   and set it accordingly. If nothing is found, use a default product
   'Fixed Cost'
- If one/several float(s) are found in the mail subject, take the last
   occurence and set it as the expense total amount.
  3/ If a product template is created, to not add taxes on it. We don't expect it
  to have additional taxes
  4/ Under Expenses -> Configuration -> Expense Product, use a simplified product
  view with only the needed fields.