These functions are moved to `inc/helper-functions.php`:
* `twentynineteen_can_show_post_thumbnail()`
* `twentynineteen_image_filters_enabled()`
* `twentynineteen_get_avatar_size()`
* `twentynineteen_is_comment_by_post_author()`
* `twentynineteen_get_discussion_data()`
* `twentynineteen_hsl_hex()`
Additionally, `twentynineteen_add_dropdown_icons()` is moved to `inc/icon-functions.php` to join `twentynineteen_nav_menu_social_icons()`.
Props akshayar, allancole, grapplerulrich.
Fixes#45984.
Built from https://develop.svn.wordpress.org/trunk@47214
git-svn-id: http://core.svn.wordpress.org/trunk@47014 1a063a9b-81f0-0310-95a4-ce76da25c4cd