Add a profile field controlling whether users want to receive email.

Not everyone wants to get emails from Review Board. This change adds a flag to
the profile to control whether users will get anything directly addressed to
them.

This is the release-1.7.x version of the change.

Testing done:
* Ran unit tests
* Tested database evolution.
* Tested the account settings.

Reviewed at https://reviews.reviewboard.org/r/5908/
Merge branch 'release-1.7.x' into release-2.0.x
Fix building API docs due to the Djblets object caching change.

Djblets's WebAPIResource now sets request._djblets_webapi_object_cache
when being called as a view. However, the webapi doc generation code
bypasses this, resulting in a breakage when building docs.

We now set this on our DummyRequest manually, in order to give the
Djblets functions something to work with.

Reviewed at https://reviews.reviewboard.org/r/5918/
Merge branch 'release-2.0.x'