Add option to disable download confirmation

Add a checkbox in the download confirmation to disable the dialog for the current user.

* [x] Add a checkbox in [template](https://github.com/4teamwork/opengever.core/blob/lk_ogip7_opt1_checkin/opengever/document/browser/download_templates/downloadconfirmation.pt)
* [x] Catch the checked checkbox in [DocumentDownload](https://github.com/4teamwork/opengever.core/blob/lk_ogip7_opt1_checkin/opengever/document/browser/download.py#L11) and save it with [ftw.dictstorage](https://github.com/4teamwork/ftw.dictstorage)
* [x] Add functionality in [DownloadConfirmation](https://github.com/4teamwork/opengever.core/blob/lk_ogip7_opt1_checkin/opengever/document/browser/download.py#L31) for
 * [x] `download_confirmation_enabled`: `boolean` (Default `true`)
 * [x] `get_download_link`: Returns `<a>` Tag?
* [x] Update views to use `get_download_link`

@phgross How do we reset the value?