Change the API for get_remote_repositories().

get_remote_repositories()'s API was using plan IDs as a way of
identifying users vs. organizations, which led to some redundancy.
For example, "private" and "public" for GitHub meant the same thing
(users). GitHub in particular was assuming certain filter types for
repositories, which would be handy to specify from a caller's point of
view.

The function has now been changed to remove the plan parameter and to
instead have a dedicated "owner_type" parameter that specifies what
"owner" refers to. The value is dependent on the hosting service, and
HostingService subclasses are expected to be smart about the defaults.

It also now takes a "filter_type" parameter that allows for filtering
repositories. Possible values are dependent on the hosting service as
well. For instance, GitHub's allowed values consist of what their API
natively allows for their "type=" parameter in repository queries.

Reviewed at https://reviews.reviewboard.org/r/5903/
Set a default for settings.AJAX_SERIAL when running unit tests.

The change to prevent extension reloading during tests needed an
accompanying trivial change to set settings.AJAX_SERIAL. This wasn't set
before, and unit tests depended on it. We were getting a working value,
though, because ExtensionManager recalculates it when loading
extensions.
Add support for API GET unit tests requiring admin logins.

The API unit test mixins now provide flags for specifying that the GET
tests require an admin account. This works just like the equivalent
tests for DELETE, POST, and PUT.

Reviewed at https://reviews.reviewboard.org/r/5912/
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 implementation was done on release-2.0.x because of the testing
infrastructure, but I also plan to backport this to release-1.7.x

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

Reviewed at https://reviews.reviewboard.org/r/5907/
Merge branch 'release-2.0.x'