Placeholders in subject were not being replaced because there was
no `.format()` called on the subject.
This commit creates a context dict that is used for both the body
and the subject. It is then replaced using `.format_map()`
Fixes https://github.com/pretix/pretix/issues/531
* added signature field -- no function yet
* added mail signature feature
* fixed style issue
* fixed problem with signature default
* added unit test for mail signatures
* added unit test for mail signatures
