Basic Teamcity agent - only supports guest authentication, default branch

Basic support for the TeamCity CI server (https://www.jetbrains.com/teamcity/) in a similar style to the Jenkins agent check.

Unfortunately I'm not very well-versed in Python testing techniques so I wasn't able to write any tests here. Help with that would certainly be appreciated.
This will be set automatically as a `source_type` in our backend with the check name, so you don't have to worry about tagging it this way.