Introduce sending of mails

Introduce sending of mails when a new orga message is saved, and when a new AKSlot is added even though the plan is already visible.

Introduce necessary basic settings for that feature (including the usage of a debug email backend for dev setups).

Sends an email with the text and a link to the corresponding AK to the contact mail address specified for a given event (implements #116 (closed)).

Sends an email with information about the slot and a link to the corresponding AK to the contact mail address specified for a given event, but only if the plan feature is used, the plan is already visible and the slot was not directly scheduled (implements #117 (closed)).

The mails are always in English, since there is no "default language" for an event and there is also no client language that could be used by the translation feature when sending mails.