[MERGE] [IMP] mail: Inbox usability improvements :

[MERGE] [IMP] mail: Inbox usability improvements :
- notficiation_email_send field, renamed into notify_email, has now 2 values: always or never, in
order to ease the choice and simplify options.
- inbox: removed 'compose a new messages or write to my followers', because those 2 options are
already available. The first one is accessible using the top-right email icon, the second one
is accessible with the 'write to my followers' text box alread present in the inbox.

bzr revid: tde@openerp.com-20140417094133-8ip7vuy6b30xlc32