minimal mail template editor for administrators

fixes https://github.com/owncloud/core/issues/7177

The current implementation is minimal in the following ways:
* only the three mail templates can be customized.
* editing templates in themes other than the `default` theme will create a .bak of the template
* clicking the 'reset' button will move an existing `.bak` file over the template file
* resetting a template of the `default` theme will delete the file from the `themes` folder and cause the hardcoded templates to be used

@MTRichards @karlitschek @PVince81 @schiesbn