BandwidthTestHandler: Generalise for multiple stream_configs per process

Goes with lhcb-datapkg/PRConfig!405 (merged) and lhcb/Moore!3285 (merged)

• Generalises the BandwidthTestHandler to deal with multiple pairs of (process, stream_config), such that we can have (spruce, wg), (spruce, wgpass) and (spruce, turcal).
• Whilst here, try harder to always get a message on Gitlab even if there are failures in the handler. Also make the copying procedure a bit dumber i.e. more foolproof.
• Adapt to upstream change of message.txt -> message.json,
• Fix a broken link where we weren't uploading the updated version of a html page,
• Factorize out functionality to helper functions a little bit.