Add support for fake SCMTools entries that Review Board does not

provide, but Power Pack does. These tools are not implemented; they

simply appear in the Tool dropdown in the "Add Repository" form when

Power Pack is not enabled.
When a fake tool is selected in the form, an advertisement for Power

Pack is shown and the form becomes unsubmittable. This is to ensure

that users don't end up getting an error when trying to use a tool that

is not actually registered in the database.
This patch also moves all of the tool information serialization into

the RepositoryForm instead of the JS template.

Ran unit tests.
Manually verified the following:

The form cannot be submitted when a fake tool is selected.
The Power Pack advertisement shows when a fake tool is selected.
The fake SCM Tool entries do not appear when Power Pack is installed.