Allow users to disable notifications

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco) Original Redmine Issue: 1435, http://dev.mconf.org/redmine/issues/1435

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Add options for the users to decide which notifications they want to receive (via email) and which they don't want to.

To allow users to disable notifications, first we need some way to show these notifications in the interface, otherwise the notifications would be lost completely (and some notifications can't be lost, such as invitations to web conferences).

See #1412 and #1043