Limit e-mail recipients when adding reviewers.

Occasionally, the only change to a review request will be to add additional
reviewers. This is especially common when a review request is first sent to a
generic group, and then people in that group make suggestions about who might
be more appropriate to review the code in question.

This change makes it so that when the only change to a review request is to
change the list of reviewers (people or groups), the e-mail that is sent is
sent only to the author of the review request (assuming that they have "own
actions" e-mails turned on), people who have starred the review request, and
the newly-added reviewers. This was requested by a user who has some very
high-volume e-mail lists, in an effort to cut down on the amount of traffic.

In the case where the change was to remove reviewers, the e-mail is sent only
to the author. In the case that the author has "own actions" e-mails turned
off, no e-mail will be sent.

Testing done:
Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/6683/
Merge branch 'release-2.0.x'
Fixed Sphinx warnings in the doc generation code.

This fixes a series of warnings from Sphinx in the auto-generating Sphinx
configuration files and our custom Sphinx extensions. These involve short
overline, short underline, and unknown target.

Testing Done:
Built the docs.

Reviewed at https://reviews.reviewboard.org/r/6688/
Make sure all Web API definitions and docs list when things were added.

This change audits the webapi definitions to make sure that every field has an
'added_in' annotation. It also audits the webapi docs to have a 'versionadded'
field for each resource that was added post-1.5.

Testing Done:
Used git blame, git log, and git tag to find the commit that added each field.
Built the docs.

Reviewed at https://reviews.reviewboard.org/r/6680/