fix deprecation warning

fixes the following:
------

DEPRECATION WARNING: It looks like you are eager loading table(s) (one of: content_plans, content_plan_taggings_947d740, content_plan_needs, content_plan_users) that are referenced in a string SQL snip
pet. For example:

    Post.includes(:comments).where("comments.title = 'foo'")

Currently, Active Record recognizes the table in the string, and knows to JOIN the comments table to the query, rather than loading comments in a separate query. However, doing this without writing a full-blown SQL parser is inherently flawed. Since we don't want to write an SQL parser, we are removing this functionality. From now on, you must explicitly tell Active Record when you are referencing a
 table from a string:

    Post.includes(:comments).where("comments.title = 'foo'").references(:comments)

If you don't rely on implicit join references you can disable the feature entirely by setting `config.active_record.disable_implicit_join_references = true`. (called from _app_views_content_plans_index
_html_slim___4490503596098976456_69909357144160 at /var/govuk/content-planner/app/views/content_plans/index.html.slim:30)