Fix step header and stage tree not sticking

[lewisbirks]

The --header-height variable doesn't exist in jenkins-core until the baseline targets 2.507.

As such an appropriate value based on the height off the breadcrumbs bar has been used as a fallback

Fixes #739

Testing done

Visual checks manually.

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[lewisbirks]

@janfaracik it would be good if you could give it a once over

[janfaracik]

@janfaracik it would be good if you could give it a once over

Will review in person - see you in a sec.