Add branch switcher to empty state of the pipeline editor
The MVP for the branch switcher will cover the basic functionality we need from the branch switcher, which is being able to load content from another branch and committing to it.
This issue covers an extra use case where the user lands on the empty state and needs to switch to another branch. For example, if the default branch doesn't have a CI file but other branches do.
Edited by Mireya Andres