The recipient building portion of send_review_mail has been

refactored out into two methods: build_recipients, which returns sets

of Users and Groups that should receive the e-mail, as well as

recipients_to_addresses, which builds sets of addresses with fewer

queries than before.
The logic of determing the recipients of an e-mail has been moved out

of send_review_mail and the signature of the function has changed to

accept the TO and CC fields as sets or lists of Users and

Groups. The logic has also been corrected to no longer send e-mails

to the following:

users who have starred a review request but are no longer in the

   review request's local site;
users in the limit_recipients_to list who are inactive; and
users in the limit_recipients_to list who are no longer part of

   the review request's local site. 

All methods that call into send_review_mail to send e-mail now must

calculate the addresses they will be sending to. This is to allow for

a future change to allow extensions to filter the TO and CC

fields of e-mails.
A large portion of this change is a style and docstring cleanup to

bring it in line with our style guidelines as well as adding numerous

new unit tests to cover all the behaviour of the two new functions

and ensure it does not regress.

Ran unit tests.