Split headliners for mississippi

[rattboi]

Currently, co-headlining acts at Mississippi are bundled together with / between them. This later confuses Last.fm, as it can't determine the correct artists.

This attempts to add a few more steps to make it clear what is happening, and splits the artists on '/'.

[demophoon]

Please rebase this off of demophoon/trello-concert-tracker#master to remove merge commits.

[rattboi]

Rebase complete