fix for old versions of jinja

[drewbanin]

The in filter was added to jinja in v2.10.0, but dbt currently has a requirement on jina >= 2.8. Some folks might not have installed dbt for a while, and jinja can fail hard with this error when the in filter is used:

Compilation Error in macro unknown (materializations/insert_by_period_materialization.sql)
  no test named 'in'
    line 51
      {%- if not '__PERIOD_FILTER__' is in sql -%}

This PR uses the built-in .find method on Python strings to avoid using partially supported jinja functionality.