File a copy of the mail in the dossier by default when sending documents from GEVER

This implements **OPT5** from [**OGIP 7** (Optimierungen User Interface)](https://docs.google.com/a/4teamwork.ch/document/d/1deP6tiDRf8_vmL0uAXwEpyLNgtBp7CbHouRnWosGYmA/edit#)

![send_documents_file_copy_in_dossier](https://cloud.githubusercontent.com/assets/405124/3483294/7b75a92c-038c-11e4-87de-7ca969cf3fa9.png)

The "**send documents**" form now contains a checkbox "**File sent mail in dossier**" (True by default). When this option is checked, the mail that just has been sent will be archived in the respective dossier. The `delivery_date` of the archived mail will be set to the current date, which implements the requirement from [**OGIP 6** (Umsetzung Öffentlichkeitsgesetz)](https://my.teamraum.com/workspaces/onegov-gever-innovation-session/improvement-proposals/ogip6-umsetzung-offentlichkeitsgesetz/view)

/cc @phgross @maethu