beta_invite email context: Use sites framework

[peritus]

This contains one fix and one improvement:

(1) Uses Site.objects.get_current() to build the invitation verify url when sending emails. This is done in a similar fashion to how django-registration does it.

This is needed if the admin site and the production site have different domains.

(2) I found an old reference to the named url pattern beta_invite_url (that's the only occurrence, I grepped the whole repo), which I updated.

[peritus]

BTW, this is how hunger/invite_email.txt looks like afterwards:

Congratulations, you've been invited!

http://{{ site.domain }}{% url 'hunger-verify' code %}

[yesimon]

I would suggest using the get_current_site(request) function instead to clean it up a bit as it does the check and returns a RequestSite automatically if the sites framework is not installed.

https://docs.djangoproject.com/en/dev/ref/contrib/sites/#hooking-into-the-current-site-from-views

[peritus]

It explicitly checks for it:

24   +    if Site._meta.installed:

.. But I'd have to check documentation on why this works.

[yesimon]

Yes the idea is not to rely on internal private properties, like Site._meta, even though it has been stable for ages. get_current_site helps keep track if there are backwards incompatible changes in the future.