Warn if retriggering a pipeline that has changed

[flosell]

Problem

After the pipeline code is changed, retriggering a build that ran with the old code might have unpredictable consequences.

Examples:

• Slightly changing the behavior of a step but leaving the overall structure and semantics the same: Probably works fine and as intended
• Completely changing the structure of the pipeline: probably visible in UI and retriggering will probably fail immediately (therefore not causing any damage)
• Changing the behavior substantially but leave the overall structure the same (e.g. adding a parameter that's passed to a deployment script): Might fail in very subtle ways (e.g. in the given example, the deployment script will still be called but the parameter was missing, possibly deploying a broken version)

Potential Solution

tbd