Remove need to use deprecated SiteMiddleware

[cspollar]

Wagtail 2.9 deprecates wagtail.core.middleware.SiteMiddleware which makes the current site object available in request.site.

References to request.site in your code should be removed; the recommended way of retrieving the current site is Site.find_for_request(request)
Source: Wagtail 2.9 Docs

There are 3 references to request.site in views.py that will need to be updated before the middleware can be removed.

This resolves #51

[Aleccc]

This change is critical, since this entire package is useless without this fix.

[drimacus182]

@tomdyson Is there any reason these simple changes cannot be merged into master? Now Wagalytics package is completely broken with new Wagtail

[xavitorne]

@tomdyson hi, sorry for disturbing but is it possible to do the merge for this issue? Thanks!

[tomdyson]

Thanks @cspollar! Sorry for the delay.