Add a GitHub plan to use another user's repository.

Review Request #9028 — Created June 20, 2017 and discarded — Latest diff uploaded

Review Board
Previously, a user could only configure a repository within the user
or an organization. With the addition of this plan, a user will be
able to configure another user's repository by entering the user's
name and desired repository.

The error messages for all plans were also updated by using
dictionaries and %s to get the user's name and repo's name, rather
than outputting a static string.

Ran unit tests with and without parameters, and had no errors.

Manually tested user:chipx86 and repository name:baseconfigs.git,
and it worked with no errors.

When the plan is tested with random inputs for both username and
repo name, we are given the correct error message.