Allow pinned Perf Plots to follow Branch start points

[epompeii]

Currently, pinned Perf Plots will always use the Branch UUIDs given to it at creation.
This may be counter intuitive when used in combination with Branch start points.

Add a "follow Branch start points" option to pinned Perf Plots that will follow the branches to their new start points and this update the UUIDs used in their query.

[epompeii]

Completed: https://bencher.dev/docs/reference/changelog/#v0416