Speed up tip migration script

[rohitpaulk]

Reticketed from gratipay/inside.gratipay.com#468 (comment)

When we wrote this script, we thought that we'd only be using it for a month or two. Now that it's here to stay - time to work on it a bit :)

The script takes so long because we're pulling in all approved teams. We could place a check in the script so that we only pull in those teams that satisfy the following criteria -

1. Team owner must have 0+ tips from Gratipay 1.0
2. Team must have zero payment_instructions that have a ctime lesser than the team's ctime (i.e. migrated tips). We use the ctime attribute to differentiate payment_instructions that were created as a result of migrated tips vs created in Gratipay 2.0

https://github.com/gratipay/gratipay.com/blob/master/bin/migrate-tips.py#L6-L10

[rohitpaulk]

!m @aandis

[aandis]

!m @rohitpaulk

[chadwhitacre]

!m @aandis @rohitpaulk 💃