Search between columns using any column header data

See https://github.com/kubernetes/test-infra/issues/4646.

Currently, click-and-drag search between columns brings up a regression search link, but only comparing the values of the 'build ID' of those columns. It'd be nice to have additional options on the menu that let you select any column header data for the comparison, so commits from other repos, or possibly a time search or similar, can be used instead. (This may require other custom URLs for regression search in different repos, or other searches).