Enables axletter mailings to a subset of subscribers

Enables axletter mailings to a subset of subscribers

* Creation of an auxiliary table, axletter_subset, which contains the list of user_ids
    to which letter shall be sent
* When sending email, sent only to those in that list and in the promotion range and who
    have subscribed to the letter
* When creating the letter, it is possible to give those mail adresses, which are internally
    converted to X.org user_ids
* No warnings when some adresses aren't recognized, ...

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>