Allow users to opt-out of their own updates.

Add `should_send_own_updates` field to Profile model. This field determines
whether or not the user should receive email updates about their own activity,
such as submitting review requests, reviewing requests, or replying to review
requests. The default value for this field is `True`.

Add an evolution, `profile_should_send_own_updates`, to add the same field to
the Profile model. This evolution sets the value for the field to the default:
`True`.

Add `should_send_own_updates` field to the Profile settings dialog. This field
is tied to the `should_send_own_updates` field on the Profile model.

Add `should_send_own_updates` method to the User model. This method is patched
on like the other methods in `accounts/models.py`.

The `send_review_email` removes the user sending the email from the set of
recipients  if they have their `should_send_own_updates` field set to False
unless they are one of the people assigned to the `people` field of the review
request.

Testing Done:
Ran evolution successfully.

Unit tests pass.

Tested with local development server. With `should_send_own_updates` set to
`False`, when a review request is created, that user only receives an email if
they are assigned as a reviewer (in the `people` field, not the `groups`
field).

Fixes bug 3316.

Reviewed at https://reviews.reviewboard.org/r/6429/