Add hostingsvcs.change_hostingserviceaccount perm to local-site admins.

On master, the unit tests for the remote repository resource are broken because
permission tests for the hostingsvcs.change_hostingserviceaccount permission
for local-site admins don't work. I'm not sure why this doesn't also break on
2.0.x, but either way this permission should be listed.

Testing done:
Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/6281/
Check for repo+commit when associating review requests with commits.

The post-commit UI had a bug where it would link revisions to review requests
even if the repository didn't match. This wasn't initially found in my testing
since almost all of my testing was with Git (where revisions are SHA-1 hashes),
but does present a problem if there are multiple Subversion repositories (where
revisions are just integers).

Testing done:
Ran unit tests.

Fixes bug 3545.

Reviewed at https://reviews.reviewboard.org/r/6282/
Refactor HTTP access out of HostingService class.

This change adds a new `HostingServiceClient` class which handles the basic
details of HTTP and JSON communication with hosting service APIs. This is
primarily just extracting the functionality out of the `HostingService` class.
Individual `HostingService`s can override this class to add additional checking
(such as GitHub's checking of rate limit headers), or add higher-level API
functionality.

Testing done:
* Ran unit tests.
* Tested post-commit review request creation.

Reviewed at https://reviews.reviewboard.org/r/5691/
Fix a bug where the body wasn't included in HTTP POSTs

This snuck in during some of the hostingsvcs refactoring. Trivial fix.