Keep history of the pipeline structure

[flosell]

Currently, LambdaCD only keeps a history of builds, not of the structure of the pipeline at this build.

If the structure of the pipeline was changed after a build happened, old build will be treated like a build with the new structure, resulting in undefined behavior: Viewing it might show wrong or missing status and output, retriggering might be broken and so on.

Therefore, LambdaCD should keep not only the output of a build but also the structure of the pipeline at that time.
Also, after a change to the pipeline retriggering of old builds should be prevented until we find a way to do so safely.

Tasks:

• Implement interface designed in #131
• Implement fallback: If we don't have a saved pipeline-structure, use the current one and display a warning
• Detect if pipeline has changed and warn when trying to retrigger (might be switched off with configuration if too annoying) (-> will be fixed in #139)