Extra migrations triggered when customizing BLOG_PLUGIN_TEMPLATE_FOLDERS

[yakky]

Description

If first BLOG_PLUGIN_TEMPLATE_FOLDERS item is not plugins, a new migration is triggered due to hardcoded value in migrations

## Steps to reproduce

Steps

1. Customize BLOG_PLUGIN_TEMPLATE_FOLDERS by setting to

   BLOG_PLUGIN_TEMPLATE_FOLDERS = (
       ('timeline', ('Vertical timeline')),  # reads from templates/djangocms_blog/vertical/
       ('masonry', ('Masonry style')),       # reads from templates/djangocms_blog/masonry/
   )

2. Run ./manage.py makemigrations djangocms_blog

Versions

Any

Expected behaviour

No new migration is created

Actual behaviour

Migration created with:

Migrations for 'djangocms_blog':
  djangocms_blog/migrations/0040_auto_20200624_0649.py
    - Alter field template_folder on authorentriesplugin
    - Alter field template_folder on genericblogplugin
    - Alter field template_folder on latestpostsplugin