Make projects onboarding beyond supported ones possible

[sr229]

Probably another failsafe for #24.

Let's have two user cases to determine the plausibility of this of needing implementation.

Case 1: I want to work on a project not supported by my janitor's instance yet

Since Janitor features a development environment, people would like to onboard to projects immediately beyond the supported projects by Janitor. An example of this would be incubation projects that would require support explicitly from Janitor which cannot be fulfilled immediately by users.

Case 2: I want Janitor to onboard my projects as easy as just adding it from the UI or appending from the URL.

Projects like Eclipse Che enables such, along with reproducible workspaces called Factories, and Gitpod as well enables project onboarding by just prepending its URL to a GitHub repository. These kinds of user experiences are beneficial for teamwork and rapid development for the user.