Log ticket purchases via usermeta

[pkevan]

Partially related to: #639

A user should be able to see what tickets for WordCamps and Events that they have previously purchased - we can probably achieve this by hooking into the purchase (and refund) flows of Camptix to store this in usermeta so that there is a way to view this.

Some of this data does exists in logs, but overall (and while logging in is required) we should store this in a way which makes it easy and efficient to retrieve this data.

Flows on tickets would include:

• purchase
• refund
• transfer

Potentially we could backfill some of this into profiles, but that can be secondary to this.

cc @renintw

[pkevan]

noting that we can probably follow similar logic to here: https://github.com/WordPress/wordcamp.org/blob/production/public_html/wp-content/mu-plugins/camptix-tweaks/camptix-tweaks.php#L335