[RTA/DPA BW tests]: Minor maintenance/UX improvements for the BandwidthTestHandler
Goes with lhcb/Moore!4248 (merged) and lhcb-datapkg/PRConfig!457 (merged)
Closes lhcb-datapkg/PRConfig#13 (closed)
See commit history/commit messages for list of changes. Generally trying to improve the user experience of the bandwidth tests.
Example new comparison pages: (they now catch when a line was removed as well as added, and report large per-line bandwidth changes even if rate doesn't change)