Branching: Add ability to prepend steps

[kahboom]

Please describe the feature that you want to propose

Add the ability to prepend a step to the first step in a branch (insert a step before the first step in a branch). This is currently not possible for non-branch steps, as you would just insert a step, so it's not necessary.

• Add prependStep function
• Add test for prependStep function
• Add button for prepending a step
• Add handling of prepending step button
• Add validation for prepending a step
• Add test for validation of prepending a step

Epic: #140