Make BandwidthTestHandler (a bit) more robust

Goes with lhcb-datapkg/PRConfig!349 (merged) and lhcb/Moore!2651 (merged).

This MR makes BandwidthTestHandler a bit more robust by:

• Not hard-coding/assuming any of the file names output by the bandwidth test. We can just upload all the files with a certain set of file extensions (we already were pretty-much uploading them all anyway), except for index.html, which of course must be present.
• Use a single function to build the comparison tables per stream and per line. Previously there was a hard-coded rates-all-lines.csv that had to be there; now all we do in assert that the per-line table has to have a Line column, and the others a Stream column. I think that's a fairly good assumption, rather that the hard-coded paths.
• Move a few blocks of duplicated code into helper functions. Move the mattermost feedback into its own function to try to reduce the size of the collectResultsExt function.