Handle multiple performances per date when importing

[jcraigk]

When importing data from Phish.net API v5, we should use show_id instead of showdate because when there's an acoustic set or separate appearance, they are listed as separate shows but need to be combined into a single sequence of tracks on Phish.in. See example https://phish.in/1999-07-30.

The convention as of this writing is use the Pre-Show (P) set designation and give more detail in taper_notes and/or admin_notes.

Related to #287