sandboxes: UI to select workflows on merge

[josephjclark]

When merging, allow me to select "all workflows" or individual workflows to merge.

In the list of workflows, we should include an icon for any workflows which have diverged

[josephjclark]

This is super important for avoiding merge conflicts. When merging workflow 1 into main, I don't want get a conflict because someone else changed workflow 2

All the backend support is done - we just need a UI