Use Session to save impersonation target

[fopina]

https://github.com/surface-security/django-impersonate/blob/2697ddaeb95be60887f6e21699330974dbbd619a/impersonate/middleware.py#L21

Instead of using a new cookie, maybe it can be stored in Session. This will also have the benefit of making it encrypted (not that it is critical anyway).

While at it, also store "original URL" where the impersonation was triggered, so once impersonation is finished it can be redirected to that URL.