Fix: insert by period for 0.19

[jtcohen6]

Extends #319, fixup of #309 (v0.6.3)

This is a:

• bug fix PR with no breaking changes — please ensure the base branch is master
• new functionality — please ensure the base branch is the latest dev/ branch
• a breaking change — please ensure the base branch is the latest dev/ branch

Description & motivation

• Update insert_by_period in a way that should be compatible for v0.19.0 and earlier versions. I'm hoping we can release this as a patch to 0.6.3, rather than a new minor version :) I tested on v0.19.0-rc1 and v0.18.1.
• Include a --full-refresh step to build test_insert_by_period twice

Checklist

• I have verified that these changes work locally on the following warehouses (Note: it's okay if you do not have access to all warehouses, this helps us understand what has been covered)
  • BigQuery
  • Postgres
  • Redshift
  • Snowflake
• I have updated the README.md (if applicable)
• I have added tests & descriptions to my models (and macros if applicable)
• I have added an entry to CHANGELOG.md